Why use classic?
Besides its free of course.
Minecraft full has creative mode which functions sort of like classic. Main difference is that it has all the blocks. Creative is just as moddable. Are you doing this out of nostalgia?
Because it feels like a gigantic feat for anyone who understands it, especially since the classic code is very very obfuscated and there are no available source code files. This means its difficult to find the required class files to modify. Plus, decompilers are never 100% accurate, so it's a pain in the butt to modify the ones you need. Then there's your problem on how you're going to add the blocks in the first place.
Classic, or creative? (these are just observations I think are appropriate)
Pros for using classic:
- free, like you said
- much more straightforward (its actually easier to add them when you get it figured out)
- holy stuff less lag
- never been done before
Cons?
- Classic's code is a bitch
- Creative has many more blocks
- Creative allows more interactive-ness availability with blocks
Let's just say its super freaking hard and it feels pretty dam good to get this all figured out. (no idea what your nostalgia is)