Looking for accessory cards

I am contemplating building a somewhat complex heating control system. I
find lots of sites that will sell me a single board computer, but not a
lot of information on what bits and pieces I might need to go with it,
nor whether it can be mixed and matched with accessories from other
suppliers.

And, on top of that, SBCs tend to be pricey - in fact the cheapest
solution to to buy a slug (NSUl2), but then you have the problem that
everything else has to come in the form of external USB-pluggable devices
and you finish up with a bird's nest :-(.

What I need is:

SBC ARM-based with 32MB Ram, 16MB flash (or, better, room to add a
memstick with just a bootloader in the onboard flash), ethernet
connection, and some bus with a well-established interface.

Additional card for A/D converter(s)

Additional card for connection to push buttons, external relays, etc.

Additional card for LCD with maybe 100x100 pixels.

Possibly wifi card.

Possible battery backup.

Plus, of course, powersupply, enlosure, etc.

I have no problem with hacking the enclosure to take it all, and to
provide bits of straightforward circuitry for sensors, pushbuttone, etc.

And I have head that the PFI bus might be sensible for the purpose.

But adding up all those cards from the sites that I have seen seems to be
a bit pricey :-( .

Suggestions, preferably for stuff obtainable in the UK, or at least in
Europe?