I just purchased an Outback Terrain 260trs, so I can't comment specifically on the 250...I looked into one, but the 260 fit our needs better.
1. Outside storage is an issue on my unit as well, although I have a small pass thru storage in the front of my unit, the opening is small though so it won't fit much. With the bicycle storage on your unit however that opens up a huge area for storage while traveling. Another option is to install a rear bumper rack for coolers ect, or a roof mounted storage container for the tow vehicle. This was possibly the biggest downside of my unit as well.
2. The vent outside isn't a huge deal for me, as I do almost all my cooking outside. The charcoal filter will work fairly well, and as with your unit, mine also has an outside vent near the kitchen. I wouldn't worry too much myself.
3.I have no issues with the construction of my unit so far, but as with most ultra lites, there are some corners cut. I notice then on my unit as well, but overall I would rate the Terrains better than most other ultra Lite units I've been in.
4. The no window in the bunk, nothing you can do there unless you cut a hole in the door and put your own window in.
5. My dump is near the rear, but I think realistically every campsite is different and some will be better than others. Have an extension ready if need be.
Another unit we considered with a Passport 2400bh. With the optional upgraded exterior (Grey as opposed to white fiberglass) it was a great unit, but the Outback had so much more seating, and features it was no brainer for me. What really sold us was the uniqueness of the Outback Terrains. Every other unit we looked at has bunks in the rear, a dinette and couch in the slide, and a queen bed up front. Every. Single. One. Seeing something different was refreshing.
Another forum member "Cotay" (he has a 250trs) helped me decide on mine, and I hope I can do the same for you. Outbackers.com has some useful insight as well.
All your concerns are valid, but I think every TT is somewhat of a rolling compromise... All will have their pros and cons. For us, the Outback Terrain just had the most pros. If you do get it, I promise you'll be happy with one.