Warning - this blah, blah, blah ended up getting wordy... (Apologies :()
If you have not owned a TT before, it can be bewildering and you can even end up making costly mistakes. You can go to an RV show and walk through what seems like hundreds of TTs, but it does not take long before they all start to look alike. TT manufacturers tend to use similar dark-ish cabinetry, wall & fabric colors. They share the same components like fridges, AC units, water heaters, exterior doors, frames, axles, etc. They don't tell you important things like say how the axle and tire weight ratings compare to what the actual weight on them will be. They emphasize dry weights in order to make them more attractive to those who don't have larger tow vehicles. It's simply so hard to differentiate between them on quality and features. To make matters worse, owners of the same make & model can have a TT full of problems while others can have few - depends on the day of the week, month, and which workers were on the line when it was built. There is no such thing as quality control. Anyway, here's a few thoughts that come to mind:
Our 1st TT was 20 ft. and had no slide. We bought it because we had a truck at the time that couldn't handle more than a 5K lb trailer. We thought it would be our forever TT. I spent a lot of time & $$ on mods & upgrades. However, after just one season, it was clear it was too small and cramped so we decided to sell it and get a better TV and longer TT with slide. We were offered a really low trade-in value at the new dealer. We bought it a year-end blowout price, but dealer's use blue book values based on production year so even tho. we only used it one season, it was considered a 2 year old RV. Tried selling it privately but no bites so in the end we sold it to my BIL. We lost a few thousand or more $$...
It might be wise to rent, borrow, steal or whatever, a TT and try one out before laying out a bunch of $$. Some suggest even buying a used one for cheap first.
If looking at "ultra-lites", know that they build them to weigh the lightest possible using the least substantial, least substantial, thinnest etc. materials. You can a bit of an idea by looking at cargo carrying capacities (CCC or NCC). Some TTs can have low CCC like 1K lbs or 3K or more. This is not a definite but can give you a sense of how much more substantial a TT's construction can be.
TT manufacturers like to play games with weights. The safest way to ensure a tow vehicle is capable is to take the advertised dry wt. and add the CCC to that to get the actual tow weight or use the GVWR rating if listed. Then take 12-13 percent of that to get the tongue wt. Your TV will need to have the payload capacity to carry that tongue wt. Don't let a salesman tell you off the cuff that your TV is just fine for a particular TT. Another thing is that some manufacturers size the axle and tire ratings to an absolute min. but that can lead to blowouts and bent axles. You can estimate the wt. on tires and axles by taking the GVWR and subtracting the calculated tongue wt. and comparing that to the load rating on the tire's sidewall. Axles will have a sticker on them that will give you their rating. TTs can weigh more on one side due to a slideout(s) which can cause overloading of tires on one side.
I would allow for 1K lbs of cargo on top of an advertised dry wt. Some suggest 1500 lbs. I weighed our current TT before and after loading it for camping and before doing any mods and upgrades and the additional wt. came to 555 lbs. If you plan to tow with one or more full holding tanks, allow for 8.3 lbs per gallon capacity. Also allow additional wt. for a battery(s) and propane tanks. Sometimes a TT can easily end up weighing close to it's GVWR rating. Ours will be over if we tow with one full holding tank (not good). If you order options, it might reduce your CCC.
Pay attention to frames. Not all are alike. Some ultra-lites use a frame that have I-beams that are made from 3 pieces of mild steel welded together that *look* like a one-piece I-beam. The 3-piece I-beams flex a lot more and can lead to serious problems in some cases. Most manufacturers have Lippert frames but a few use a BAL Ultraframe that is bolted together instead of welded and they have a good reputation. IIRC, Jayco uses them. Nash and Outdoors RV use the same heavy duty frame built in-house. You might want to look at these to see how they compare to Lippert frames. Even tho. different brands/models of TT are similar length and wt., Lippert frames are not built the same. Some manufacturers spec. stronger frames.
If you have a choice, I'd opt for a TT with a plywood floor and not OSB. Look closely at how cabinets, dinette seating, under-bed storage are built. I've seen some terribly built stuff.
Most TTs come with ST ("special trailer") tires. These have a max. speed rating of 65 mph and need more care (never tow under-inflated or overloaded) otherwise blowouts can result. Most ST tires are made in China and have a reputation for premature blowouts. In some cases, a TT manufacturer may offer an upgrade to a different brand with a higher load rating (for ex., LRC to LRD). The tires are often a generic Chinese brand with questionable quality. It is recommended to have at least 10 percent reserve load capacity and more is better to prevent blowouts (we have something like 30 percent on our LRD Marathon tires).
Some manufacturers are better at warranty support while some have a bad reputation. Some TT brands/models have forums specifically for them and it would be a good idea to peruse to look for quality issues and how a dealer or manufacturer handled warranty repairs. Similarly, there are good dealers and terrible ones. Some are quite inept at doing repairs. See what you can find out about a dealer you might end up having to use. It is common in the RV industry that when you have something that needs repair, you get passed off onto a third party vendor to fight it out with. Some manufacturers and some dealers are better at helping you and some do everything they can to avoid dealing with it.
Don't get taken in by "bling" and miss the things that really count. One dealer even told us that the manufacturers of the RVs they sell focus on bling and not substance. If you have narrowed down to a short list, spend time sitting down inside them, look closely what storage is like, etc. Is the TV in a decent location that is comfortable and visible. Is there enough shelving and storage in the kitchen for dishes, food, etc. Most kitchen counters have almost no place to work on and for appliances but there are some that do have more space. Are the sewer hookup, low point and FW drains in a decently accessible location. Functional things can be overlooked.
This is a bit of a primer off the top of my head. There's a whole lot more things you could look for but would take pages. Everyone on this forum will have various helpful suggestions and opinions from seasoned RV users. Most important this is to search everywhere on the internet you can. Ask questions as you learn more. Ignore what a salesperson tells you. Go to an RV show and spend lots of time in, around, under the units and looking inside cabinetry.
Good luck!