Well, at some point you need to trust SOMEONE. No one is going to mod the system with you standing over them watching, that just isn't realistic.
I was a bit worried at first, before I sent my Neo cart system to Jeff for the 4X mod (also known as "the works"). But the solution is to send your system to him, insured and with delivery confirmation. His price for the 4X mod (s-video, stereo, arcade debug, and two external buttons that are territory and arcade/home toggles) is very reasonable, and his work is, in a word, outstanding. You can trust him absolutely, I had my system modded and he did a fantastic job. In fact, I have another (backup) cart system that I will also send to Jeff for the 4X mod.
Also, I have the necessary carts to send him for a Metal Slug conversion, which I will be doing within the next few days.
We are all very lucky to have Jeff doing these mods and conversions. If I were you, I'd throw your doubt in the trash can and get moving, have him do the work while you still can.