Recommendations wanted

Hello All,
I am in the very early stages of planning a new product that will use
distributed SBCs with digital I/O cards for control purposes.  I am a
programmer and will be writing the control software.

In the past I have used low cost SBCs from Acrosser running DOS with simple
serial communications.
Things have obviously moved on in recent years and I'd like to take
advantage of a more versatile, yet robust OS.  Hence I'm interested in
Linux, although I know absolutely nothing about it,  I've only ever worked
with Dos and Windows.
I'd like to use TCP/IP sockets and perhaps one or two useful services such
as a FTP server.  I don't require a graphical interface.

There are so many versions of Linux, SBC manufacturers and digital IO cards
that I don't know where to begin looking.  I'm hoping that people from this
group may be able to point me towards proven combinations of hardware and
software that meet my requirements.

My requirements are simple:
1) Low cost.  Both of hardware and OS.
2) Compatibility between OS, SBC and the IO cards.
3) Reliability
4) The availability of programming tools.

I don't need much processing power, if I can get away with using a 386 based
system then I'll do it!
Does anyone have any recommendations?

Regards,
Warwick Stock
(The Win32 programmer looking to defect)