Sadly, it's all down to £'s, but you'll soon get used to that as a boat owner.
If you're on a tight budget then obviously you're probably gonna have to be happy with whatever's out there e.g. the 585 mentioned. Yes, there will probably be 'a few' things wrong with it / not to your liking but it'll get you out on the water and if the hull is sound then a great platform to start with.
If you've got a modest budget e.g. the 585 price plus the same again, then I think you could do an awful lot of good to that boat to get it up to a v good condition and, maybe more importantly, to a spec / layout that you / your family needs.
If money's no object, buy new to exactly how you want it.
I could'nt have got (at the time) anything like what I have now for the money I've spent, and more importantly for me the layout. With a small bit of effort (towing it to Mike's place from Glasgow) I was happy to give a 21yr old RIB a new lease of life, as he was ! Yes, I'd love a shiny new 4 stroke hanging off the transom but I'm sure that'll come, especially if I use the 'it's for safety, darling' reasoning as mentioned in another recent thread !!! Having said that there's absolutely nothing wrong with the 2 stroke lump I've got (outboard, not wife 😉
I'd definitely do it all again. It obviously will have to be bigger though !!!