mudbug,
1) In my opinion 100 Mhz should be ok, it wont seem any faster than 7.5.1 but it wont be too much of a drag on the system.
2) I would max the ram upto 64mb. Then you will probably get away without virtual memory and should run alot more smoothly.
3) hard disk drivers... If there's any way you can make a back up just in case the hard disk becomes corrupted then you'll be fine. (I've updated the Hard disk drivers on my 6320 plenty of times with no problems)
Just a little tip... System 7.5.1 is about the WORST OS you could run on your mac... it will be very buggy because it's primarily designed for non PPC macs. If you don't updated to OS 9, you should at least get the free update to 7.5.5 (I agree with Herr Newton on that one)
I'm running OS 8.6 on my performa 6320 (120 mhz) with 40mb of ram (Doubled with ram doubler to 80mb) i've updated the hard disk drivers dozerns oftimes, without any problems. From what i've experienced you should be ok.
Hope this helps.
Eddie.