In that case, find what you can as cheap as you can...
My first S-100 system was an old (faulty - but repaired) Cromemco Blitzbus backplane with a North Star disk controller and a CPU/MEM card that I bought from somewhere cheap. Plugged them all together and they worked...
When exactly was that?
OP never explained whether he has, or is willing to learn, repair skills. And also acquire sufficient test equipment to integrate and debug a random collection of components. Or whether he's ready to tackle the challenges of old floppy media and drives. If he wants to do that, and learn the intricacies, from scratch then more power to him :-}. Such a bootstrapping education is valuable :-}. But that may not be what he's looking for ... TANSTAAFL.
Even a S100 backplane isn't cheap anymore, and look at the cost of connectors to populate a new backplane PCB :-{. Collecting the components, including case, to assemble-your-own isn't going to save much in material cost at the end-of-the-trip. Add costs for test & repair equipment.
IMO the offer from
@smp is more-than-fair. If the OP simply wants an "S-100 bus computer", including removable media, that "just works" (and from a reputable restorer) then the marketplace pricing is what-it-is. If budget is a problem then there's really no alternative to accumulating bits-and-pieces over time (the installment plan) and investing a fair amount of sweat-equity as he proceeds. Occasionally relatively inexpensive clunkers show up on eBay (maybe $500 and not including media/drives) with the usual
caveat emptors.
Unless he has the interest, time, and patience to "learn the trade" IMO the assemble-your-own route is more likely to lead to tears than cost savings.
OP doesn't mention his plans for a terminal -- Teletype? Glass teletype? -- either of those would be era-appropriate, not a window on a laptop. Neither come cheap.
Is the OP intending to run, say, Cromix rather than CP/M? (Not on the Altair, for sure.)
Just sayin' that the OP objective is vague, under-constrained, and (IMO) incomplete if he desires a complete operational capability (of some sort) in the end.
An alternative strategy is to simply move directly to a modern CP/M-capable implementation with built-in media and serial and/or USB support, of which there are a plethora available at attractive price-points. Use your laptop/desktop for the terminal. Or get a "Enhanced VT 100 Terminal Emulator Board" at
https://www.legacypixels.com/other/index.html and add a VGA display for a close approximation to a legacy terminal using "real hardware".
There's also the nice
https://adwaterandstir.com/altair/ that you can put on your shelf and admire. OP can "can pick up in the realm of hundreds of dollars". Includes the joy of assembly. No one will know that the result doesn't have an S100 backplane inside.
