In the child seat hierarchy these are essentially the first two sizes that you need:
Maxi Cosi Cabriofix like this and Maxi Cosi Easyfix Isofix base like this
Prefer not to split these; £60 for the seat and base together.
And then the Maxi Cosi Priorifix (which has the Isofix gubbins attached) like his £70
£120 if you want the lot
Pics here
These seats have reviewed and tested very well with the likes of Which? and others. They are in good, sound condition and have not been dropped or involved in any accidents. Some cosmetic scuffs on the Cabriofix. I'd be happy for my daughter to still use them if only she still fitted in them, grrr. They are good colurs too, quite rare, and far better IMO than black/blue/pink that most seats are. Collection (or GONADS) preferred as I imagine carriage would be expensive... (Isofix bases are heavy)
Shortly there will be a Quinny buggy which the Cabriofix will attach too, and I think we still have the carry cot that goes on it and a few Quinny accessories but they are not down from the loft/cleaned up yet... Oh, and a couple of decent stairgates too.