Static caravans can be remarkably spacious, and, like boats, have lots of really clever storage.
Unless you are used to a rambling old farmhouse or a mansion, I wouldn't be too sure you need two caravans for just two of you. Especially if you are keeping on the old place for a while.
If it were me, I'd be more inclined to think I'd want a second space as a workshop or something
Heating and condensation would be the two major issues. If you don't want gas, you could fit a small log burner - they really heat up the space, you don't need a very big one! If you can get it warm enough that you can leave a window ajar (or similar) for ventilation, that'll help with the condensation too. And if you get one with a flat top / cooking ring on top, you can do one-pot all-day stews and so on
The other main issue is mud coming in, and muddy boots storage. So some sort of covered porch, even an awning (made sufficiently wind-ripping-off-proof) to let you take boots off outside would make a big difference.
There are two types of static caravan. The holiday type are thin-skinned, and lose heat over winter, so may need insulation (depending on how many rooms and the size and reach of any log burner or other heating.) The 'park home' type are designed for year-round living and are properly insulated. For 2-3 years, if you can afford it, I'd try to get one of the latter.