I would have thought that you estimate for the hull and tubes was a bit low, more like £10,000 but if all you want is a 150 then I guess £7000 might buy you an Optimax.
I thought that Ribeye was an expensive as a buy it in bits to fit at home kit. The Shakespeare will offer you the cheapest hull build with a decent hull and set of tubes on. Then you're into the RIBcraft and Osprey terratory (both very sea worthy RIBs), I looked at both these and decided on the Osprey due to many endorsments and the flexibility of the builder to make modifications, they also glassed in a Console from another boat manufacturer without question and modified seat bases without any major cost.
But if you seriously want a Sea worthy RIB at a bargain price, look at the Shakespeare.
Have a look Here
Then PM Neil Harvey (hope you don't mind Neil) and ask his advice