probably a stupid question, but the more I think about it, the more I feel like I want to get into this build thing.
What parts are you looking for to build an AR?
Are all uppers/lowers compatible with each other?
Recommendations on equipment to seek out or avoid???
I would say start with building a complete lower. With an AR15, any upper should fit any lower. So you could build your own lower and then buy a complete upper. If you want a different upper, it's easier to sell a built upper vs trying to sell an upper you built.
Parts are all standard, all you have to watch out for is if your lower is to Colt M4 specs or not. Only difference is roll pin size. Oh, you also have to pay attention if you go with a mil-spec buffer tube or not. It's a different size tube so you just need to know which you have for your stock.
For tools, you need a small hammer, a roll pin punch set, a stock wrench, and masking tape. Oh, and a small 2x4 scrap piece of wood. I think that's all I use to build the lower. Lots of videos on building one on Youtube. Watch a few of them and decide which looks like the easiest way to do it.
Building your own lets you go with the look and parts you want to start off with. Otherwise you are changing out parts on a built rifle. You can try to stay within budget with a build, but like most stuff you build, you often have to double the budget in the end because you always find cool stuff that you want.
Not really much to avoid that I can think of. I don't care for polymer AR lowers, others are fine with them. Some cool looking stocks are really heavy. They look cool and work well, but not something you want to carry around due to weight. You can buy lower parts kits without a fire control group (trigger) if you want a better trigger in the gun. Saves a little money if you want a nicer trigger.