We bought a 2nd hand dinghy from a guy in St Lucia who restored them. Probably not there any more but his prices were at least double what you'd expect to pay over here. If you can possibly take it with you, do so.
The various shipping companies, such as Peters and May, are often sending containers or part loads to hubs such as St Lucia (end of the ARC) and Antigua. I shipped two windsurfers that way to Antigua and cost wasn't excessive. Much cheaper than buying locally.
If I was in the market for a small RHIB, I would go for one with aluminium hulls by AB - significantly lighter than similar sized fibreglass versions.
New ones 'might' cost a bit less than say a new Avon or Zodiac in Europe (especially when you factor in shipping charges).
Re outboard motors, Yamaha generally seem to be the most popular, followed by Johnson / Evinrude.
You 'might' be able to buy a secondhand dinghy and outboard out here - but be careful, it might well have been stolen off another yacht.
Be aware that RHIBs and (especially) outboards are very desirable things to steal, and even locking cables are not much of a deterrent if somebody is equipped with bolt croppers - and it is not unknown for folk to find that somebody has taken a chain saw to their transom to 'tief' the engine.
Budget marine is the main caribbean agent for AB so you can see prices. You will likely win 20% discount off those prices. AB have gurt big 17-18 inch sponsons and fine in the sunshine. Carib bout the same praps not as cheap - main agent for carib is Island Water World and both they and budget have dealers up and down the islands.
I would be surprised ed if ribs are not cheaper here in st martin thn in europe, really?
Outboards last year price was 1650usd for a 15hp enduro 2-stroke yamaha, and 2600usd for 25hp - i bort the 25hp.
Be careful bout the aluminium - i agree with bajan BUT i bought one and immediately part-exed it - it was very flittery and felt about to fly off sideways, erk! Also quite noisy. Also was a touch over short-shaft sized. Also very deep - the plastic ones eg my 12VS has fwd locker good for getting on/off.
There are a lot of used Caribes around in the WIndwards and Leewards as the likes of Moorings and Sunsail use them now instead of the hard dinks they used to provide. It would be worth your while asking around when you get the Caribbean but how you avoid being the receiver of stolen goods is another matter altogether.
They were being nicked at the rate of one a week in Trini for a while last summer.