Man o' man, where to start...
They can be quite accurate but remember, they were designed to be accurate enough to hit a man and be rugged/reliable at the same time. Your "typical" FAL normally get approx 1.5-2 MOA as long as the shooter does his job. Accurized models will see group sizes of 1 MOA, maybe slightly less.
A store bought FAL will run you approx $1200 for a DSA model. Yes, DSA is the best if you go this route.
How hard they are to assemble depends on your mechanical inclination.. I've been a backyard mechanic most of my adult life and had zero problems building the 2 FALS I own. You can expect to save quite a bit of money building one yourself, even more if you decide to build many.
Which tools you need is dependent on how professional of a build you plan to make it. You can get by with less, but to do a truly pain free, professional build you will need the following:
1. Receiver wrench (A must)
2. Barrel vise (Nice but not absolutely necessary)
3. Headspace sizing rods (A must)
4. Headspace gauges (Go and NoGo) (A must)
5. Barrel TDC tool (Nice but not absolutely necessary)
6. Buttstock tool (Nice but not absolutely necessary)
7. Gas nut spanner (Nice but not absolutely necessary)
I am assuming you have all the other basics such as a good strong bench vise, punches/drifts, mallets, screwdrivers, etc.