The tweaking potential of Naim preamps is formidable. Building your own power supplies, or tweaking secondhand preamps offers extremely high performance relatively cheaply. If buying a preamp which you think you will want to mod (ie, multichannel regulators etc) then go for the 42 or 62 incarnations. Having most of the components on the mother board leaves plenty of room to mount regulators inside the box. The 32 and 72 type preamps don't have a great deal of room inside them to play with, although you can mount little LM317LZ regulators actually on each slot-in board which is nice.
I also feel old 250 power amps are a bit of a bargain too. You can get Naim to give them a full service, and replace power supply caps etc, such that you end up with a very good power amp relatively cheaply.
Very old Naxos are good value if you intend to replace components and tweak them etc. Often they will only be a few tens of pounds.