Change the tweeter first- the midbass second. The HK midrange is actually quite good- and to clarify that's the 2.5" speaker next to the tweeter in the coupes tweeter/midrange assembly.
Look for a mounting depth of 2.7" or less. 6" woofer is the easiest if you don't want to have to dremel the doorcard plastic. 6.5" will fit.