You can watch both movies as stand alones, but you will miss out on a lot of the character development stuff since they kinda assume you're already familiar with the cast.
I recommend you watch the first OVA (the 7 episode one) then watch the movies. If you still want more, watch the TV series then the second OVA. The second OVA comes after the series. The series is pretty much a retelling of the first OVA with a whole hell of a lot more stuff.
Hope that helped. Patlabor is a favorite of mine. If you do watch the first OVA, it might not start how you expect, but keep with it. It changes and becomes more serious as it goes on.