Zodiac Pro 420 or 7-man (they changed the name) is a good choice, but it's no Avon Searider. Zodiac use Strongan Duotex material (basically their version of PVC). It's got a shallow to mid V hull, so expect it to slap in rough chop. On the plus side, it's a good looking RIB, relatively lightweight, huge accessory list, (but beware Zodiac is notoriously expensive), and pretty capable for most tasks.
Most of the models out there will have a rear bench seat and console. You can fit 25 litres of fuel under the console and another 25 litre tank in the bench seat. There's also a bow locker for an anchor and chain and you can still get after-market bow cushions.
I use to run a 2007 model from new, and a Yamaha 40hp 2-stroke engine (model 40V), also enough room to squeeze on a Tohatsu or Mariner 3.5hp auxilliary on a bracket.
The hull is rated for 50hp, but its fine with 40hp. It's comfortable with 2-up but we had 4-up on a few ocassions. A decent unbraked trailer like Indespension, and it's easy to launch and retrieve.
Down-side: no hull cabling, so remote controls, fuel lines and any wiring will run down the side of the deck. Actually okay on a 4.2m RIB. Watch for spider cracks on the gelcoat of consoles. Shallow V hull, so your backside will be in mid air in anything rough.
Plus-side: the tubes slide off for cleaning. It uses a unique hull rail system that tethers at the bow and stern. My model had a bow roller for retrieving anchor lines which I thought was neat.
Price wise, if it's been garaged, then don't let PVC put you off. Chances are these boats may have led a fairly sheltered life, well certainly in Blighty given the short season. I'd expect to pay around £5-6k for a good example. The engine and trailer are key to this, so be particular. I'd be looking at boats up to 10 years old as a guide.
If your budget extends a little, then a Pro 500 or 9-man is a better option powered by 50-60hp, price up to £7-9k, and can still be garaged.
This is a few years ago running from Gardenstown to Rosehearty. Apologies in advance for the music.