the marauder uses spyder threaded barrels, i recommend the j&j cermaic, great barrel and its cheap.
you could get a bottom line stock and mod a front grip(expansion chamber, regulator, gas thru grip...etc) and new asa on it. this is done by getting a raptor vertical adaptor and cutting the old asa off the bottom of the grip sand it down and drill 2 holes and put the stock and then a new asa.
what i would do is face the tank forward it would ballance out the weight of the market with the stock
or you could get a gas thru stock, it will screw in to the asa and you need to use a remote line with the tank.