3.5Ghz or more if you want to be sure Blitz and Gauntlet Legends will run. A 64-Bit OS and MAME build make a big difference on those games.
In the DIP switch setting for Blitz, you can reduce the resolution which helps a lot if your processor falls short.
You could get by with 2.8-3Ghz for the rest of the games you mentioned depending on the generation of processor.
IIRC, the full CHD set for MAME was over 250GB, but less than 20GB of it was playable.
Get the full set and then just copy over the games that work to your machine.
I have a 500GB drive in my cab that is limited to arcade and arcade style PC games.
2TB seems to be the standard for people who get into the disc based consoles and want video previews for everything.
There is a lot of new stuff out there that just might blow your mind if you've been away for 10 years.

Check out Supermodel and Demul emulators for late 90's games. They'll require a good video card in addition to a fast processor.
There's also the Taito Type X games which have been hacked to run on a standard PC, no emulator needed.
Look all those up on youtube to get psyched up to spend money on a fast processor and spend hours configuring emulators.
