I agree with Mr. Hermit. Regarding art covers, I just go with simple buckram and I am conservative with colours and lettering. I find there is a timeless appeal to plain hardcovers with just some lettering on the spine, like your collection carries a similar intellectual weight as a library of 19th century classics.
If you are in the US, Houchen is really good and makes the whole process painless. Go to their site and read the instructions they have there for comic binding.
As for number of issues per volume, it depends on whether you cut them up to remove ads and back covers or you get the entire issue bound (Smythe binding). If the former, you can get 25-30 pretty easily but I recommend closer to 20. I have the first 36 issues of New Teen Titans bound up in two volumes of 18 issues each and they are perfect for reading. Just use some good judgement - stack your issues after prepping them, factor in the thickness of the covers and go from there. It's really just personal preference.
Notes on prep: for DFAB, which is what I imagine you'll mostly use for old comics, remove the staples, cut out what ads you can and remove the back covers. Use a boxcutter razor knife. Then stack everything between two pieces of cardboard (backing boards of the type used when you bag comics are perfect). Secure everything with rubber bands to make a firm block of paper. Then affix the bindery's form to it with tape. This is the form that specifies colours, text, etc. You lay out what you want directly on the form.
For Smythe binding, just remove the staples and put the books together into a block as above.