If a build is what you want to do, then start with a DSA or an Entreprise Receiver. Cost is $299-$450. Both are great, DSA is better but costs more.
FN FAL Metric Receivers
FAL-SA58 Upper Receivers-D S Arms
Then buy a kit. My suggestion is to buy a kit without the barrel. Numerous sources for kits are out there, but DSA and Entreprise both have them for good prices and they are known quality sources (at least to me).
If you buy the kit without the barrel, I would recommend this unit. I have not yet used one, but I am seriously considering re-barreling both of my FAL rifles with this unit:
SA58 FAL 21 Medium Contour Chrome Moly Barrel, .308 Cal. - 001H21-D S Arms
I have owned Israeli Heavy Barreled rifles in the past, and they tend to hold groups well even after 50+ rounds. The standard "pencil barrel" tends to wander after 20 rounds or so. Just the nature of the beast. If you like to target shoot, and you have the discipline to let your barrel cool down between groups, then the Pencil Barrel is fine.
FAL rifles are GREAT!!!! With a little work, they are very accurate out to 800 + meters. This means a torso sized target, and 2-3" groups fired in sets of 5 rounds. Optics are doable, and the DSA replacement receiver covers (picatinney mounts) work VERY well. The lesser expensive ones work really well too, but they are not as burly as the DSA ones. Expect to use locktite or equivalent on the cheaper ones. The cheaper ones are made of aluminum and the DSA may be steel depending on the model, read close!!!! They do sell aluminum too!
Good Luck. Post pics so that we can enjoy your triumph as well!!!!!