These days, there are about a million ways to collect old Spirit stories. You can search on eBay for the original newspaper inserts. Those are fun, and they're not actually all that expensive. You can spend the big bucks and find him in the pages of Police Comics, as well as eponymous titles published by Quality and Fiction House, respectively. There's also the long running Kitchen Sink reprints, the short running Harvey series from the 60s. Obsessives (like me) continue to fork over money for the fancy schmancy glossy, hardcover treatment the stories receive in the DC Archives series. There are many more versions as well, but I do have to work at some point.
For my money, however, you just can't beat the Warren magazines from the 70s. Jim Warren worked with Will Eisner and produced 16 issues, mainly in black and white featuring new covers. These are awesome. The strip works so well in black and white - and they've done so many shades of grey that the artwork seems so beautifully texture. Some of this would be lost in the Kitchen Sink magazine reprints, as the reproduction quality was inferior. This is also a great format, as the larger page size is perfect for Eisner's layouts and splash pages.
Most of my copies (I think I've got 14 of the 16) have yellowing pages, but they've got a bit of a pinkish hue to them and it just seems to add to the overall atmosphere. All of this and the great Captain Company ads make for a very pleasurable and nostalgic reading experience. The great news is that they're still cheap - I picked up a nice copy of #16 for $2.99 the other day. So if you need a Spirit fix, but are a bit scared of investing in the Archives, try tracking down a few issues of the Warren run. I think you'll be pleasantly surprised.