It really depends on what you want to do with the boat. The F24, and F28 do not have standing head room and are really designed mainly as racers. The F27 is more a cruising design but IIRC, it doesn't have standing headroom either. The accommodations are relatively spartan—usually having only a porta-potty and camping stove.
The F31's cabin is not much bigger than that of the Telstar 28 I own, and is the first of the Farrier designs I'd consider for long-term cruising/sailing. My friend and his wife took theirs on their honeymoon cruise to the Caribbean.
The two major complaints I have with the Farrier folding system are the fact that it requires tools to fold/unfold the amas and that when the amas are retracted, the topsides are submerged—which is why almost all farrier designed trimarans are kept on moorings.
If you're going to get a custom one built, I'd highly recommend going with Airex foam for the hull and Divinylcell for the deck, and using a couple layers of kevlar in the hull laminates, on the exterior for abrasion resistance and on the interior for puncture/impact resistance.
If you have questions, let me know.