Mad started out as a standard comic in 1952 and then changed to a magazine format in 1955. Are you talking about the standard comic issues? They are pretty valuable.
You may know that comic value is highly dependent on condition. I will give you a list from the current comic book price guide of values for the Mad comic issues in three conditions: Good, Fine, and Near Mint. Good is your standard used and read comic with creases, minor tears, tanning pages, etc. but still complete and pretty solid.
Fine is for a comic with only one or two minor flaws, like a couple inch cover crease or a 1/2" tear. Near Mint is a comic that looks like it just came off the newsstand with no noticeable flaws. #1: 520/1560/7000, #2: 140/420/1900m #3,4: 85/255/1150, #5: 148/444/2000, #6-11: 61/183/825, #12-15: 48/144/650, #16-23: 38/115/525
Of course comics could be graded in between these values (or below Good if the comic is pretty beat up). These are also retail prices. If you seek to sell comics to a dealer, you will probably only get 30 to 50% of these amounts. Selling on eBay could get you into the 40 to 70% range with good scans and descriptions.
EBay is also a good place to go to see how well these comics are selling. Old issues of Mad show up pretty regularly under Collectibles/Comics/Golden Age. You can do searches of completed auctions and watch current auctions to see how they are doing.
You may know that Mad was originally a comic book for the first 23 issues. The main comic book price guide, called the Overstreet Comic Book Price Guide, does list both the comic book and magazine sized issues with prices, information on artists, and occasionally also information on stories or spoofs of particular interest. It also has information on the Mad Special and Mad Follies issues.
Issues. You may not want to buy the whole book, which even in soft cover sells for $25, for two or three pages of information. If so, you can find them in many libraries and just make a few copies. There was also a "Complete Mad" CD-Rom issued a few years back. I don't know how complete it is, but I'm sure it would be a great resource.
Mad Magazines from the very early 1960s have a price guide value in the $50 to $70 range if in newsstand-new condition. If in your typical well-read and enjoyed condition, this would drop to $6-7 each. Prices continue to drop so that in perfect shape/well-read shape, by the mid-1960s to late 1960s: $30-50/$4-5 each, 1970s: $10-25/$2-3 each
However, these are the price guide prices, which are what shops try to get at retail. If you were trying to sell them yourself on eBay or to a comic shop, I don't think you would find a lot of interest in this time period unless they are in like-new condition. Even then, you would probably only get 1/3 to 1/2 of the prices listed above at best.
If these are in truly perfect condition, they have a guide value of around $10 each. Even minor flaws would put them much lower. However, the market is pretty soft for Mad Magazines of this period. Looking at those sold on eBay in the last month, it looks like you'd probably be doing well to get $40 or $50 for the lot of them. You will probably find very little interest at local comic stores, but it might be worth a call or two.
As far as the comics go, the various publishers do not sell back issues as far as I'm aware. However, there are certainly a large number of comic retailers that sell back issues on the Internet. I carry all of the titles you mentioned.
While comic book publishers have reissued many titles in book format, that is not nearly as cheap or convenient as the CD/DVD format you suggest. Why not on CDs? It may be because of concerns about putting things in digital format, where they can be more readily copied. Or it may be because it seems that most fans want to have an actual comic page in front of them rather than read it on a computer screen.
You can also get a CD with all issues of Mad Magazine: see the Totally Mad collection available on Amazon and elsewhere. Otherwise, there are collectors that trade scans of comic issues or post them online for download but I have little knowledge of this. It probably goes without saying that this violates copyright laws.
Though the comics companies generally don't sell back issues (though Mad /Cracked might, I haven't read them since my elementary school days), there are generally scads of back issues available at any comic's store (not bookstores, but comics dealers -look in your local yellow pages or online). Most back issues are reasonably priced compared to new books (though they may be 10 times the cover price or more). Expect to spend a couple dollars on most. Also there are comics aplenty on eBay, just search for the titles you like and see what pops up.
I recommend copy of the Overstreet Comics Price Guide available at most bookstores. Most of the titles you mentioned had books out in the years you listed, though Spawn wasn't available in 1988, I don't think.