This does not make sense. Trying to turn a fastener the wrong way has nothing to do with the right tools. If you’d had an impact driver and tried to turn it the wrong way, you might have been even worse off.
Further you don’t want an impact driver. Impact driver is for production speed, typically fasteners around 100 ft lbs or less, not just two hard to get off bolts which would benefit from a right angle impact wrench.
You probably couldn’t get a straight impact wrench into that space unless a stubby air tool or right angle.
If your wrench slips, it’s a sign you need more precise wrenches or sockets, or your bolt was so severely rusted that you tried to use the next size up (or standard instead of metric, or the last person who touched those bolts rounded them off). An impact driver or wrench will do nothing to solve those problems.
Now the odd question “best… at Harbor Freight”. Why? That’s like saying tell me the best of the worst which makes no sense unless you live in a remote isolated area and Harbor Freight is all there is, that you don’t live in a part of the world where Amazon delivers either, nor any other major tool chains or websites.
As mentioned by JeepersCreepers, you didn’t state whether you are looking for an air or electric tool. No idea what your friend meant by universal that twists in different angles, that would be an attachment bought separately, has nothing to do with which wrench to pick.
Frankly I think you need a good quality new socket set instead. Brake caliper bolts are not particularly difficult to get off with the right socket (or torx, hex head, etc depending on vehicle) and ratchet.
The “2 nuts” part is also a little odd without knowing what vehicle since the caliper is generally held on with bolts not nuts and not common for them to be reverse threaded. I would consider getting shop manual for your vehicle, doing some Youtube video searches for specific repair jobs, and asking in an owners’ forum for that vehicle. Odd things that deviate from standard practices like having reverse threads, would be mentioned in details related to that repair.
Sorry I didn’t answer your question but I think it was the wrong question, lol.