yeh on areas where theres no damp youll be fine.. just have to give the walls a real good brush down with a stiff brush first then a good soak with some weak pva to give your dabbing a chance..
just look out for the loose bits mate.. old walls can be a mare to dab over..
all in the prep..
i wouldnt bother dabbing mr board meself..if theyre not going to be wet areas e.g. bathrooms, ensuites,kitchens etc.. then you wont need em..
if theres a damp problem you should really solve the problem first, mr boards wont really help you..
dabbing over a damp wall will give you a headache in a matter of days mate.. in fact youll prolly find the plaster when finished wont dry over the damp areas..
just bear in mind that all it takes is a week of heavy rain and problems like that grow into big problems..
not having actually seen the ground level over dpc at the back of the house i cant really advise properly, theres all sorts of things to consider when addressing damp mate.. levels, dpc failure, bridging, ventilation..
to nip back to otions 1 and 2 though.. a scratch coat of render will do two things...
1) it'll make your walls easy to dab onto... and
2) more rendering practice...
all good..
dont expect render to cure damp problems though.. the waterproofer just tries to stop any water left in the wall from manifesting itself internally.. any continued passage of water (e.g. raised levels) will cause it to fail eventually..
also a good idea if 2 coat rendering over an EX damp wall to make sure your waterproofers got salt repellant/neutraliser in it..