Even though you want to by used (a good move) I would suggest attending an RV show - just to get a feel for floorplans and to gather data about various brands - the good, bad, and ugly.
You don't mention if you have kids or not. If you do, I recommend a unit with bunk beds - and if possible a full sized bed for the two of you.
If you are talking of only using it on weekends with a mix of dry camping and full hook ups almost any amount of fresh water and holding tanks will work. Just make sure that you have more holding tank capacity than fresh water.
When looking at rigs, take your time, sit a spell, pretend you are camping, try out the seating areas, the beds, the bathroom. Yes I am serious about the bathroom. Sit on the toilet, stand in the shower etc. Do you have enough room for everything??
As far as brands go there are so many to choose from and all have their share of lemons. Since Fleetwood has the lion's share of the market you will probably find lots of units out there. Another brand that is good, but rare in the used market is Arctic Fox or Nash. There are some out there (check www.afnash.com
for some listings). It is not uncommon to find units from the 90's that are in real good shape.
Once you start looking at real potential units, come back here for more info on how to spot a lemon.