u can put in an equalizer valve on an axle that doesn't have them. just make sure that u have it supported when u put them in and that ur air lines are rated for the pressure. just make sure u only do it on one axle for each equalizer valve.
if u are gonna go with a semi-convo, i would suggest a 379 peterbilt with a 60 series detroit engine and a 13 speed eaton fuller tranny. the 13 peed will give u a really low granny gear for climbing hills. a w-900 kenworth would do well too. just stay away from volvo, those things are built for comfort and not durability and reliability. i used to drive a 379 pete with a detroit in it and that sucker had over a million miles on and it still ran like a top.