1, The choice of seat is your yours, any seat can be used, but you have to remember that whatever seat you use, needs to be waterproof, pvc,vinyl,etc. So you may have to add the cost of recovering the seat to your budget.
2, Also you need to think about what sort of seat adjustments are required,(mine was a fixed back seat and the trike built to my size so no runners were needed), do you need it to slide forward or backwards? if so make sure you get some runners when buy the seat, or if ripping it out of a car take all the mechanisms (electrics seats can be used, BUT you would need to make the electrics waterproof,and they tend to be bulkier seats, so may not fit a lot of trike’s.
3, there are a few mechanisms you can use. (other than making one from scratch)
a, The mechanism off a stannah type stair lift.
b, The mechanism off a mobility scooter.
c, You can buy one that fits into a car that’s swivels the car seat out, which would apply the same to a trike, But having never seen or used one of these? i do not know if it is manual or electric or how much space it needs?
d, Some Bus (drivers) seats also have a swivel mechanism, and you may be able to get these cheap from a specialist breakers, (also some industrial machinery must use them , such as forklifts etc. but you would have to do research on this.)
4, Remember if you are going to go this route, then you need enough space around the seat (front back and sides) to allow it to swivel, and i know its obvious, but also enough room for your legs to swing round, to be able to resume the normal riding position, no point in having this feature, if the last bit is going to be a struggle.
5,NOTE IMPORTANT: The swivel mechanism you use, must be able to lock the seat in place, when in the forward facing position, (this is an MOT/MSVA requirement)
6, The location of the swivel mechanism is important as well, some swivel from the centre of the seat (most do) but the stairlift mechanism swivels from the side of the seat, (this is what i had) this allows the chair to swivel further away from the trike, allowing greater access.
7, The fitting/installing of the swivel mechanism, is going to be different on every trike, so there is no way i can say how to fit it, all i say is get a competent welder to do the work (especially as your life would depend on it, if the weld breaks).
8, You would also need to clarify the current position on whether a seatbelt is legally required, for your specific vehicles design.
9, One further point worthy of note, is the support of your legs whilst seated on a trike, (especially for those with a disability.) I had some adjustable leg supports made out of ‘Drain Pipe’ and some lengths of threaded bar. (see Picture Below) this was a very effective method of supporting your legs, especially if you are in the classic stretched out ‘Chopper Position’ obviously these can be made out of anything, and for a more professional finish drain pipe is not ideal :-).