A new kit that's any use will be £700 upwards for a complete kit of everything you need for a sequential system (don't touch a mixer system).
For £200 it will either be second hand, incomplete or possibly a mixer system. In either case, it's to be avoided, I would say.
Regarding fitting cost, I would say you'd be very lucky to find a fitter who would touch a second hand system or one he didn't supply himself.
Bear in mind also that a second hand system might not comply with the current code of practice so it may not be possible to certify it.
To supply and fit a new LPG system, prices range from £1000 to £2500. I'd be suspicious of their workmanship at the bottom end of that range. In fact, I'd be very careful to check out whoever was going to do an install for me, regardless of the cost. Many "UKLPG professionals" are actually far from it, IMHO.

Kevin