Be sure to check out the sponsor pages. Heavy Throttle's site is jam packed full of goodies for building an SR. HT also offers complete engines package/front clips if your wallet is too heavy for your liking.

They also have an invaluable FAQ section, and other useful sections like their OEM SR parts list. I usually suggest that people start by reading everything on sites like HT to give you a basis for important pricing/availability on SR-specific parts.
Also, if you do decide to swap engines, remember that wiring is key. There are a few good wiring diagrams floating around. If you need a diagram, try running a google search on "s13 wiring" "s13sr20" and so on. These basic searches, along with just about any search containing s13/sr20/wiring--combination will give you enough reading material to last you a day of two.
Welcome aboard and good luck with the car! Be sure to keep us posted on your car's progress, regardless of what engine you run.