For a car, you don't need much. Car trailers are really cheap like that. You might want to check around. Sometimes it is almost the same costs if you build it yourself or buying it. Then you don't have to get it inspected, you don't have the hassle of building it (unless you are a very bored person) and you have a warranty. There are ton's of companies out there that make trailers like that, so the market can be cut throat (good for consumer, bad for retailer
) I can think of three companies that are out of Texas (or close to) off of the top of my head. Big Tex, PJ and Loadtrail. We don't sell them for a few reasons. Mainly because freight trom TX to ND is killer. We have a MFG in ND here that builds trailers just as nice or even better (stronger built in my opinion). Since they are built in TX you have very low freight costs. I do not see why you can't get into a basic model for under 2k.
If you insist on building one here is just a crash course of materials. Frame materials (some use heavy 3-4" angle others use 3-4" channel), crossmembers most use usually 2" angle ( you need enough for 16" on center, some go further apart, but you have a much more solid trailer if you go 16") You need channel to build the tongue at minimum 4". Material for ramps (usually 60" long)
You need axles, two 3500# axles will be fine. Most states require at least one have brakes, some states require brakes on both. Suspension (spring, hangers, equalizers, shackle straps, shackle bolts - That is if you do not go with Torsion Axles) You need a coupler, you can probably get by with a 2" coupler, would reccomend a 2 5/16" though. Lights (two tail lights, and at least 4 clearance lights - check with local laws on that though). Fenders, tires/wheels, lumber (for floor when you decide to put it in, go with 2x8" green treat (Easy to install, strong and longer lasting) Floor screws, Primer, Paint, Safety Chains, Break-Away Batter (might not be required for inspection in TX, I don't know), Electrical Wire and Connectors, Band-Aids, Beer and Time
It can be done... and has been done lots of times with patience. Just sit down and draw out a plan of the trailer you want. Length, width etc. and then go price shopping for parts.
I don't want to discourage you from building one, I am all for building your own. I just don't want you to end up like some guys here that think they can build their own but by the time they are set and done they could have bought one with a warranty for less. Just make sure you do your homework. If you have more questions be sure to ask!!