with what you can do with the adwater altair 8800 you won't even want to deal with a real one.
It's very pretty and all, but technically it's an emulator in a pretty case. And, you know, different strokes for different folks, if that doesn't bother you then I'm sure it's a lot less hassle to set up (and a metric **sload less money to acquire) than an actual 8800 replica, let alone an original unit, with similar capabilities. Where I suppose I'd argue it's not the best choice in the world is if you're actually wanting to build (with hope of understanding) an *actual* 8-bit computer from the ground up, vs. just learning how to *use* one. I mean, ultimately it's kind of a cosplay, albeit a pretty convincing one.
I'd agree that RC2014 is a solid recommendation. Conceptually it's pretty comparable to an Altair-style S-100 machine, in being completely modular and thereby giving you the freedom to mix-and-match boards to build whatever you want, be it a minimal embedded system with a serial console or a full-fledged CP/M machine with extended memory, various video console options, etc. The advantage it has over going straight to an S100 replica is the bus and cards are much smaller and cheaper, having been designed around the reality that in the modern era you can, for instance, fit all the memory you might need for an 8-bit computer into a couple chips vs. multiple 5x10" boards. There are also a ton of software options in terms of ROM languages, cross-compilers, and OSes bootable from ROM/flash (CP/M and others) to fit the hardware, so if your ambitions don't extend as far as writing all your software from scratch there's a lot to keep you busy.
Personally I'd steer away from Apple I replicas. Brass tacks, there just isn't that much you can do with one; the computer is an extremely crude design with a dog slow console (which most kits don't even replicate in hardware, they include a microcontroller programmed to handle terminal functions, or just expose a serial port instead), there are only a few dozen programs for it, and most of them are pretty trival (even the BASIC doesn't really work, missing things like a proper SAVE command). Unless you're specifically a huge Apple fanboy there are better options for simple 6502 SBCs.
Grant Searle's webpage has already been mentioned, it you want to just dive right in and do it from scratch on a breadboard that's another option. RC2014 in its most basic forms borrows a lot from Grant's '9 chip CP/M' and simpler BASIC-only version. (A "default" BASIC for Z80-based RC2014 machines is Grant's hacked version of Microsoft's BASIC for the NASCOM 2 computer, as used in these machines.) Another option, if you want a complete tutorial, might be to watch Ben Eater's YouTube series on building a 6502 computer from scratch; he walks through all the moving parts in gruesome detail.
Another thing to note, I guess, is that some vintage computers are pretty hacker friendly with accessible bus connectors, so if you want to start out with learning how to interface and build simple peripherals you could do worse than lay hands on, say, a Commodore PET, a TRS-80, Apple II(+), or a PC/XT compatible. ISA prototyping cards are easily found and electrically speaking it's a pretty forgiving bus. (Although of course you are risking letting the magic smoke out of the computer if you mess up. This is where older is better; most of what you could blow up in an IBM 5160 or TRS-80 is generic TTL and can be replaced with modern parts, while with newer machines you're kind of SOL if you blow up a proprietary chipset component.)
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