some other options which may be of use are below since your deal has deal has went pete tong
some are better buys than others but it was a quick look at what is about in your budget and ages you are looking at.
there is also a couple of nice boats in the for sale section on here
2005 Ribcraft 585 Rib - Suzuki DF115 | eBay (the tubes are a bit faded but they look recoverable with TLC/rib revive and elbow grease)
Ribeye 550 playtime 2002 with a Mariner Optimax 115 2006 rigid inflatable rib | eBay (strange layout but may suit OP? i'm sure playtimes are PVC tubes unless spec'd otherwise from factory, need to do your own digging to confirm)
Ribeye S650 classic with Bramber Trailer | eBay (looks tidy, not sure about tube material on classic's though)
2008 Humber 6m Destroyer Rib, 40 hours use from new, Full service history | eBay
2009 Avon 580 Adventure RIB - Suzuki DF150 hp Fourstroke - Just In.72 hrs | eBay (i'm not sure what the 580 hull is like, it is 100% not the same as the 560 i had which would slam)
Ribcraft - 585 RIBs and Inflatable Boats for sale in Hampshire, South East | Boats and Outboards (looks very clean, weird colour combo so may get a decent discount?)
Ribeye - A600 RIBs and Inflatable Boats for sale in Hampshire, South East | Boats and Outboards
638 - 650 RIBs and Inflatable Boats for sale in Hampshire, South East | Boats and Outboards
Ribcraft - 585 Rib Honda BF90 Outboard RIBs and Inflatable Boats for sale in Devon, South West | Boats and Outboards (90hp isn't ideal on this hull)
Ribcraft - 585 RIBs and Inflatable Boats for sale in Devon, South West | Boats and Outboards
cheers
