If I may make a suggestion - there's a vessel out there called the
Donovan Class Interplanetary Vehicle (DCIV) - under my addons page. I gave a pretty realistic weight for the thing (2000t), plus a similar amount of fuel mass (2200t). There's a lot of stuff in the addon page, but the big thing that makes it a realistic possibility is that it runs on water (It's a steam engine, sort of - read the readme.) What you can do, of course, is refill it anywhere that's got water.
That being said, I think I got the handling and weight ratios right - I actually somewhat used the same values that Dig Gil mentioned in his post way above to figure out the mass. A bonus to this is that it's nowhere near as horrible if you want to dock a ship to it that isn't one of those specific landers - you can actually perform an autoburn without flailing wildly because you've got something the same weight as the entire ship hanging off to one side.
Note: Eventually, it -will- be UMMU compatible, but it isn't yet. Put your ships into safe mode of some description. ^_^
Other Note: Yes, I'm pimping my addon. I don't care - it's better than the deepstar.