A .50 Caliber mounted on the roof would be more effective. This means you must have a sunroof, so you can have easy access when needed.
I hope this doesn't send you on a "wild goose chase", but I saw a silver 123 car that looked good from the street for sale on 303 in Grand Prairie. If your interested in chasing it, or maybe know someone who lives in that area to run it down for you, send an email, and I'll try to give you the best directions for where I saw it.
Good luck with finding a solid car. I'm sure you'll come up with one.
Have a great day,
'01 C Class, Six Speed
'84 Euro 240D, 523K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in