Welcome to the forums. 290's are indeed hard to find. 280s and 310's are more common but also more popular for the very reasons you are interested in the smaller, single rear axle units. 325's and 345's are certainly more available on the market most of the time. Not sure why - none of us knows how many were made but the general assumption around here is that there were more 345's made than the others.
A few things to consider regarding size and floorplan. You didn't mention how you will be using the MH. As a 345 owner I'm amazed that we began our search looking at 280's. With three kids and two dogs and a couple of yearly trips in the 2000 mile round trip range, I can't imaging a smaller floorplan.
It's always easier to change a floorplan slightly to meet your needs but you can't "add two feet on" later if your 280 seems too small for your lifestyle. On the other hand you don't need much or travel light then a 280 might be perfect. If you haven't already found it a great resource for floorplan comparisons can be found at Fred's Class A Airstream website:
As far as maintenance goes: this depends more on the condition of the MH than anything else. Obviously the longer untis has more systems to deal with (2 roof airs, larger refers, two furnaces...etc.)....which to will require more maintenance but also affers redundancy IMHO.
It also has a lot to do with what you are planning to take on yourself. If you are handy and can work on the rig yourself you can save a lot of money. Chassis parts are pretty cheap and plentiful. Coach parts are pretty much off the shelf from Camping World or you local RV dealership. Things like AC units, generators, etc all require repair/replacement and if they fail at once can easily exceed the cost of a rebuild on your 454.
Good luck with your search. I can tell you they are a blast to own and get a lot of attention.