Since you will be towing a trailer, I would recommend you consider the BMW OEM hitch. I used the OEM hitch and wiring harness, and don't regret it at all. Even if you do go aftermarket, look at the BMW wiring harness (you can buy it with the hitch or separately). If the dealer is that expensive to install it, take it to any competent repair shop, preferably one you know or have a referral for. It is several hours work and whether they have done one before or not, they should be able to do it fine.

If you are even reasonably handy, do it yourself using the excellent instructions posted here, or get a friend to help out. You will need a few TORX bits, but everything else is standard mechanic's tools.

Lots of similar posts, the search function will show them.

Good luck.

__________________
2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue