Slighty belated in the thread welcome to ribnet!
As someone who bought a rib for £800 twice and got a few seasons of pretty much pain free use from one & a total engine rebuild & trailer replacement out the other I hopefully can shed some light on the "project" myths...
The first is that just 'coz its old doesn't mean it's dead. Having said that, if you are new to the rib / outboard world be very wary - there is rarely a genuine "super cheap" deal out there and if you aren't sure what to look for or it's history and you donlt have someone whao knows with you, expect surprises.
It's worth having a wee search on here for "old ribs" or "prioject" to get a feel for what can go wrong (and what it takes to fix).
- What's your engine knowledge like? If you can change the piston rings in a 16V car engine with your eyes shut then you are already a good way to being able to sort a wobbly (i.e cheap!) engine.... (I phrased that illustratively!)
Also if you wait to the end of the season, and look for separate boat & hull you are likley to get a bit more for your £ - people generally like packages, so fishing for two separate bits becomes a pain. Use that to your advantage! Likewise at end of the season people are disposing of this year's toy and there are generally less bidders....
Other things to search ebay for are the Avon Workboat and Supersport (up to 5m so potentially lots of space & can handle some big engines should you ever want to "upgrade" the hull to a RIB!) - If the seller has used the boat's name in the listing instead of "inflatable" you'd be amazed how many fall below the average buyer's radar.....
Other way of lookign at it -
Buy something old & decrepit - then do a staggered upgrade - E.g buy cheap Sib/25Hp - then trade hull in for a 4m searider, swap the engine across. A few years later take the 25 off the SR & hang a 40 or 50 on the back. A few years later put the 25 back on, sell it as a package & use the profits to fund a bigger hull on which you hang the 50. FF a few years & punt / PX the 50 for a 90 or buy another 50 & run twins...... You get the idea.
You will hear a lot of B*ll*x on here about "too underpowered". Most here would scoff at 25Hp on the back of a 4m Rib, but most dinghy sailing clubs use that very combo for rescue boats - almost always with 2 aboard....
Doing 40 knots & having power to spare is one thing. You'd be amazed how little it takes to get 18-20 knots out of most hulls.
Neeldess to say my post contains some generalisations, but hopefully gives you some food for thought.