As was said already, you will probably need the attenuator with an outdoor antenna, especially on these GRE (GRE inside) radios, that have hot front ends.
Now, with that said, while you may be happy with att on, and it may suit you (which is all that really matters I suppose), the 106 will not match the BCT15 for analog reception most likely. So you might want to still pick one up for that outdoor antenna.
Look, straight out, I have owned most of the cool scanners made in the last 18 or so years, and have been playing with them for about 30 years, and I have found that the BCT15 is the best analog scanner ever made, at least in my testing, in my unique set of conditions. Mileage may well vary, but just a heads up on this.
I have done EXTENSIVE comparison testing and found the following:
Best base scanner for 800 mhz P25 (most big agencies around me): PSR-600 / Pro-197 (best for simulcast / best digital voice decoding)
Best digtal & analog portable scanner: PSR-500 / Pro-106 (best digital / good on analog)
Best analog base: BCT15 (not even close - it rules - no attenuation needed - great filtering - sensitive)
Best analog only portable: unknown - testing not done - I have to have digital on my portables
Now, if anyone has done testing on the 164 vs the 162, I would say that would give you a pretty good idea on the analog portable comparison, but I can't say for sure. I have this sneaking suspicion though, that the uniden made 162 will do better on analog stuff (especially vhf high), than the GRE made 164, based only upon the superior performance of the BCT15 in analog testing (with an emphasis on VHF High).
Oddly enough, unidens that combine digital and analog do not have the same high level of analog (especially VHF High) performance.
All of this is based purely upon testing that I did, with my antennas, in my unique situation, with my methods. This isn't scientific of course, and I will be first to say so. I will also be the first to say that in your unique situation, results might be different. Just sharing my experiences. I think they do have some value since I put quite a great deal of effort in to them, and tried hard to be objective. Perhaps they would be of the most value to those also living in a high RF environment like myself.