I would definitely go with the phantom center as well. You have nothing to loose trying this first. If your budget is that tight, then it seems to be waste of your hard earned money buying any centre you can afford just to have one. What will having a cheaper centre that is not designed to play with your mains add to your system?
Order the M3’s set them up and if you can borrow a centre from a friend. Then flip back and forth (between phantom and centre) to see which you prefer or if timber matching is an issue for you…it might not be.
As for timber matching, basically it's speakers that have the same sound characteristics, designed to be used together for seamless soundstage.
Good luck
