Congratulations! What year model are you purchasing? Any idea how many miles are on it and what it was used for? Did they routinely have a heavy motorcycle in the back? Did you ever figure out what that modification was in the front compartment? We have the Onan 6.5KV Propane Generator in that spot.
Having a spare tire depends on how long(miles) of trips you plan to take away from civilization and if you will be very diligent in maintaining your tires at correct pressures as well as replacing them every couple of years.(Trailer tires have a one year warranty, not based on mileage.) When this rig is loaded to maximum gross, one is pushing the limit of the axle capacity. I have gone to 8K leaf springs and higher weight rated tires as we do routinely carry a big touring bike in the back. We have Good Sam Roadside and had a blowout a couple years ago. They came out and efficiently changed to the spare so we could go another 65 miles to next town that had a decent tire shop. The tow company can bring out a tire to sell to you and change it at their emergency price point. However, some require taking the wheel back to their shop and putting on a new tire while your rig sits on the side of the road. I would recommend you carry the spare. Since the spare carrier is up underneath you can get any steel wheel the correct size with the correct tire for the short amount of time you'll have it on the rig. I suspect any RV boneyard would have something as it's a common 16 inch size. Our spare that came with it is a regular steel trailer wheel. We had a good experience with Good Sam Roadside so if you're planning some long trips in remote areas, I think it's worth the investment.
As for the rig itself, I'd check the age of the tires, there is a date on them, last time the bearings were repacked, age and condition of the battery, condition of the roof, and take a whiff of the black tank through the open toilet and front galley tank through the left side kitchen sink to ensure they have been properly maintained. New tires and a battery are not a big cost to a dealer and won't break the sale if you don't like what you see. They stopped making this particular model in 2012 but I still have mine, highly modified, and we use it quite a bit. If you have any specific questions, fire away.