It is probably best to buy at least a 486. At a minimum you need 4MB to run Linux. More than that is nice if you plan to run several X applications at once and want to do some compiling at the same time. You will also need space beyond that for mail, source code, and user space. No matter how much disk you get your needs will grow to fill the space.
SCSI disks are desirable.
Having a SCSI interface in your computer is more flexible than IDE or EIDE and with Linux tends to be faster. All three will work just fine. If you get a SCSI interface for use with your hard drive, save yourself some hassle and get a SCSI CD-ROM drive as well if you intend to buy a CD drive at all. This will save you trying to find compatible CD interface cards. Find one that has the price and features you want and then check the XFree86 Howto to see if it is supported.