As you have never flown a VP chances are most aircraft fitted with a VP will be a little more complicated that you are accustom. A VP in itself is straight forward but the aircraft to which it is fitted will be different and managment of the aircraft will contain some new elements. Chances are an instructor will end up wanting to do a flight or two with you. Of course you will be able to do a least two hours in a complex single for every one in a MEP so it may be money well spent in terms of getting use to VPs.
If the cost is less important personally I would incorporate it as part of my MEP, because on the assumption you dont have much time in complex types you are likely to find a twin will be a reasonably substantial "step up". Moreover you will also deal with retrac. gear whereas there are plenty of singles with VP props with welded u/c.
There is a significant transition from non complex singles to twins both in terms of speed and cockpit managment. Having gained your MEP you will find your skills increase significantly and flying pretty much any SEP will seem pretty straight forward in comparison - tailwheel aside - in respect of which much fun will still be there to await you!!
PS - to answer your second question I have done plenty of "check" flights on group aircraft with pilots without a VP sign off - in fact you dont have too got back that far and these sign offs were not required so we just checked out the pilot on a new type. With luck an instructor will ask you a few questions to ensure you have a reasonable knowledge of VP props and will sing your log book without further ado, if you have done a conversion onto type with a group member.