does anyone have any suggestions on what model/make of boiler to
use, and whether i should bother with the extra expense of a
condensing boiler?

Vaillant are good, but a little expensive. Ideal Isar is good and very
cheap. You should go for condensing types, but you must remember to bully
the installer on two points. Firstly, the flue must be well away from any
windows, in excess of the minimums because they do produce a lot of visible
steam. Secondly, you must plumb the condensate drain into a proper drain, as
it does produce a lot of water. Many installers will play down the amount of
steam and (rather corrosive) water produced. They do this so they can spend
a lot less time bodging it rather than doing it right.

I have had some quotes for £1790 - £4000.

1790 is too cheap. I bet the pipes would be surface mounted everywhere or
your floorboards left like offcuts in a sawmill. Unless this person comes
seriously recommended by a friend you trust to know what he is talking
about, it is very suspicious.

Christian.