£6k is a small budget for a 5.5m+ rib in my opinion so I wouldn't worry unnecessarily about whether it's 2-stroke or not. At this kind of budget, the condition of the boat will dictate just what you get for your money.
You could go smaller, younger and with 4-stroke but then you'll end up with something that's not necessarily as good a sea-keeping boat as you'd envisaged.
I'm only throwing this into the mix to show what £6,700 buys you. Searider 6m (workhorse of the oil industry) with twin Yamaha 60hp outboards.
https://www.findafishingboat.com/avon/ad-88529