What benchmark test do I look for?
Iometer would be
among the best, with drive latency measurements at certain queue depths, but that doesn't quite hit the mark.
It is difficult to measure what you want. A lot of work is going into it, on multiple OSes and applications, but generalized benchmarks for it are hard (DBMSes are about the only class of applications where it's not a fairly new idea). The problem is that you're not looking for how many X you can do each second, but how often some processing of X takes much longer than average, and then when it happens, find what causes it to happen; having started out with the assumption that the average processing time is, "fast enough."
If you were me what processor and how much memory would you get?
Buying new, an Intel Core branded CPU, 4th generation (mostly on account of the IGP compatibility and software support, not processor performance). On a more meager budget, look into an AMD A8 (but the budget differences are getting very slim, as of late, with sales/bundles). Then, get double the RAM you need (since no video card is needed, get it as 2 DIMMs), and an SSD big enough that you won't fill it all the way up.
If buying rather than building, a low-end video card would be fine with an older CPU (Radeon HD x450, or Geforce GT x10 or x20, have been standard for business PCs for some years, now, FI, but aren't generally needed with the 4th-gen CPUs). But, you'll be hard-pressed to find a big OEM box with a nice SSD, that's not otherwise a powerhouse you don't want or need. In that case, expect to replace the drive, at the very least.
In fact, if you can find an off-lease business PC that takes DDR3, and has a Windows 7 license (most have refurb special Win7 Home) for <$200, sprucing it up with a new SSD and light RAM upgrade can be cheaper/better than building. In that case, too, try whatever video it comes with, but if has issues with your programs or displays, a cheap video card might be in order.