Have a look through the RIB section on here, some interesting stuff & we're now into the time of year when a lot more boats will come onto the market & prices drop:
Ribs inflatables for sale - FAFB
As with everything, you MUST view them & if you aren't sure what you're looking at then get someone who does to go with you. If in any doubt walk away.
Don't forget there are a lot of scammers so be wary.
(I wouldn't necessarily disregard some of the hardboats either - you might find more with the side by side seating you want)