A CD-ROM will not boot a 68000-based machine (Plus, Classic, SE). I'm not sure on the SE/30 off the top of my head but I think it works fine on a Classic II. If there's other programs I'm not aware of that enable CD booting on those older Macs let me know so I can add it to my knowledge database.
An original Classic can boot from its ROM. Hold down COMMAND-OPTION-X-O before you get to the Happy Mac. You'll get System 6.0.3 with network functions. There are also hidden credits in here if you use a program like ResEdit or DiskTop to poke around the folders of this disk. (You also get the brightness CDEV, but it's actually from System 6.0.6 and the icon looks a little different from 6.0.7/6.0. 8) .
As far as RAM is concerned, you're going to need a minimum of 2MB for System 7. I don't recommend 2MB if you're going to actually do decent work on it, you'll want at least 2.5MB or preferably 4MB. If you absolutely must run System 7 under 2MB or even 2.5MB, be sure you strip out unnecessary extensions, control panels, fonts, and other addons.
Any 68000 based Plus, SE, or Classic can accept as much as 4MB of RAM. Don't go by what is written on the back of the machine if you're looking to see how much RAM is in there--many Pluses were marked "Macintosh Plus 1MB" but some were upgraded. SEs are the same way.
You're more likely to find extra RAM in a Classic than any 68000 due to the RAM card that most Classic owners purchased. Most often you'll find Classics with 2MB or 4MB. SEs are hit or miss but there are a good deal that have been upgraded. The majority of Pluses still seem to be stuck at 1MB unless they used an external hard drive at some point or used to run System 7.
There's also the external hard drive factor. Most external enclosures are fine, but the drives often are not. If you need a good drive, pull a Quantum ProDrive LPS (more reliable than the EPS) from an early 1990s Mac. They were commonly seen in the LC and LCII in either 40MB or 80MB capacities.
(I use an 80MB LPS that I took out of an LCII in an Apple 20SC enclosure on my Plus and it works great--it's pretty fast as far as Plus SCSI drives go and has more than enough space even for my decked out copy of System 6).