NMO? - as in the new Motorola type fitting - is that what you mean renonation?
To answer you first question - min distance spacing between 2 NMO type antennas: in a receive only situation, no, without taking into consideration a whole bunch of other factors the antenna fitting type is going to have next to zero impact on performance. However, there may (and I say may) be practical considerations to keep in mind - like mechanical loading/stress on the body panel, which will come down to antenna element length and speed of vehicle (!!??). - all very hypothetical, but you asked, so theres my opinion. In short, I wouldn't be too concerned.
The best reply has to be a comment made by another member over the last few days regards placement of antenna's on a car: place them where you think best.
Best NMO type for 50 - 500Mhz: if I've understood you correctly, you are putting the fitting/fixing method of the antenna to the car, first - then performance charcteristics(?). The point to keep in mind about ALL broadband antenna's (and 50Mhz - 500Mhz is very broadband) is that they are comprimise antenna's i.e. to get any antenna to work across such a wide band of frequencies, performance sacrifices have to be made at various points in the covered bandwidth ... and just about any mobile antenna could be adapted fitting wise to be an NMO type fitting, if thats what you wanted. So don;t choose an antenna just because of it's fitting mechanism to the car.
Some antenna designers will comprimise the lower end of that 50-500Mhz bandwidth, to squeeze the whole lot in, whereas other antenna designers will take a different approach and comprimise the upper end of the bandwidth to squeeze in the lower half of coverage ... and another designer may choose to sacrifice performance in the mid-range to obtain coverage of both the upper and lower ranges in the bandwidth.
The general rule will be: the greater the bandwidth, the greater the sacrifices will be at the extreme ends of the total bandwidth.
The only way to evaluate broadband antennas is to do your reserach and find an antenna with the total bandwidth you wish to have covered, establish which parts of the bandwidth covered are the areas in which you are going to be carrying out most of your monitoring, and then choose an antenna design that performs best over the bandwidth that is of greatest interest to you in day to day monitoring.
What may help in your search on the internet, is to narrow the search down by using a term such as "receive only broadband mobile antenna". A lot of the antennas folk use are actually antennas built for both Rx and Tx - and that limits the avaliable possibilities quite a lot.
There are a lot of options but there is going to be no single panacea
Got around $3000 to spare - try this mobile antenna :
R&S®HE055 Antenna (Rohde & Schwarz UK - Products - Radiomonitoring - Antennas) - it's not an NMO type, but you ain't going to get better at any price in a mobile antenna, for the bandwidth you have in mind. They really do work well
