So we probably looking for something which is novice friendly, comfortable and reasonably efficient for longer rides with hills.
You may find the first and third relatively hard to resolve. They're all fantastically comfortable compared to upwrongs, but you may have a (possibly medically dictated?) preference for a certain type of seat and steering arrangements. Under-seat steering is probably the best for least stress on the hands/arms.
My understanding is that most bent bikes/trikes are one size fits all, but can require lengthening/shortening chain.
Mostly. If you're particularly big or small, there are certain models that just aren't going to work. High bikes may be limited to the small wheel versions if you've got short legs, for example. ICE do a wider, reinforced version of one of their trikes for larger riders, and so on.
Hardshell seats are quite size-specific (though some have a limited amount of adjustment, and you can muck about with foam to get a perfect fit). Mesh seats are more vague (but do tend to ventilate better), but more adjustable.
Is it possible to set a recumbent bike or trike so that it can easily be used by 2 riders of differing leg lengths. I.e. can you use some sort of chain tensioning device?
Chain tensioners for boom adjustment are available from a couple of manufacturers (certainly ICE and Hase do them). If it's a small difference, you may get away with running the chain slightly slack in certain gear combinations for the shorter rider and/or the taller one being careful not to engage big:big (barakta and I manage this with a 30mm boom extension difference on her ICE Sprint). Another option would be a couple of quick links and an appropriate extra length of chain to splice in. It all depends on how quick and easy you need the changeover to be.
Bacchetta style leg length adjustment moves the seat rather than the boom, avoiding chain issues.
2nd question is the relative merits of trike or low, mid or high bike. Apologies if this ha been discussed before.
Trikes are easy to ride (zero skill required if you keep it below 20mph on corners), and extremely safe (very hard to fall off - even on ice or with a front wheel blowout; awesome braking; drivers give you a wide berth even by recumbent standards) but inherently slower due to the rolling resistance of that extra wheel and all the other bits and bobs in the airstream. Getting trikes on trains is prohibitively annoying (most operators will refuse to carry them), but folding trikes tend to fit in cars better than DF bicycles.
High vs mid/low bikes is going to be dominated by things like leg length and personal preference, but all else being equal high bikes are going to be better for group riding with uprights (better drafting, less mud in your face, that sort of thing). High bikes enable the use of larger wheels, which may be desirable, and of course being higher up means you can see more. (Don't worry about being seen in traffic - any recumbent is better seen than a DF bike, just by nature of being unusual.) Low bikes get an easier time with the wind.
My general advice to darkside newbies would be to plan to change your mind about what you actually want a year or two down the line when you have more experience. Buying one bike/trike to do everything works at least as badly with recumbents as it does with uprights, and what's easy to ride and reliable tends not to be what's most efficient for audax-style riding.
Second the advice about a visit to DTek (his range of stock and knowledge is second to none), but if he starts being useless at returning phone calls etc, don't hesitate to go elsewhere.
Good luck!
