Nobody else has chimed in yet, so I'll summarize the useful advice I've seen.
1) When buying a rig someone else built, there is always a question of how well it was done. Sounds like you're new, so finding someone that knows their way around a modded Jeep to inspect it before you buy is a good idea. A less than quality build can cost a lot of money to fix.
2) If you're not mechanically inclined, have a mechanic check out the other bits too, to minimize surprises. Maybe this is the same person as #1. Leaks, signs that the odometer reading doesn't match the other wear, weird stuff in the fluids. Brand new fluids (could be a bandaid to cover things up). All the standard used car stuff, with an extra dose of paranoia, since a modded rig probably wasn't driven by grandma only to church on sunny Sundays on perfectly smooth asphalt roads.
3) If the price seems too good to be true, it is.
I don't know if that is the case with this specific one, but this one probably won't be the last one you look at.
4) Test drive, test drive, test drive. Even before you have someone check out the mechanicals. I looked at over a dozen WJs before I bought mine. One of them, the doors and back hatch didn't close right. Another was badly out of alignment (and too overpriced to deal with looking deeper to see if an alignment was all it needed). A third didn't make it out of the parking lot before it was misfiring so bad that it was undriveable. And on, and on. A good test drive and close look at it yourself can save you the expense of 1 and 2. Post in the appropriate forums for the model you're looking at, asking about common issues and how to spot them. For example, there is a diagnostics procedure you can do on WJs to pull any codes that are present, and another one for the hvac system to pull any codes it has. Useful info to have, though don't put too much confidence in these types of things if they are clean, because codes can be reset.
5) You are buying a used vehicle, so don't expect perfection. However, pre-modded vehicles have more wear if they were used (and an extensively modded rig probably was), even if the mods were done to a high standard. There are no guarantees in life, but you might find yourself better off finding something closer to stock and adding mods as you need them. Stock Wranglers are fairly capable.
You didn't say exactly what you're interested in, and depending on your definition of hitting the trails, a stock wrangler may do it for you. Before you buy anything, find a Jeep club in your area and chat with them about what is required in your area, and what is overkill.
If you aren't sure, starting stock isn't bad. I bought my WJ, because I wasn't sure exactly what I wanted, and it was cheaper than a Wrangler as a starting point to figure that out. Turns out that it is plenty for what I wanted. A lift and big tires and all kinds of crazy stuff is really awesome to have. But, if you don't NEED them, they are costing you money just having them. Big tires cost more money; and some mods can increase wear on the other components, which will lead you to higher costs of maintenance. Bigger tires can be harder on the axles. A lift changes the driveline angle and can lead to more worn ujoints. Just two things I've seen around the forums more than once. Someone with more experience can probably give you a laundry list of stuff.
If you know you'll definitely want all the mods, then just be careful about checking things out before you buy. And go find a jeep club in your area, because they can probably point you to a good shop and/or help you evaluate particular vehicles you're interested in.