i ride both road and MTB and i'd say that its well worth getting an entry level road bike to train on if you are serious about getting the best results from your road training
i've got a giant OCR3 which works fine for road training and won't break the bank, in fact i've had less trouble with my Sora equipped OCR than my training partner has had with their Dura Ace equipped TCR1 !!
so while some new wheels and slicks may help you out a bit if you want to ride road get a road bike and get all the advantages of big gears and light frames etc