I would personally use it as a QT. 10g is the min for any sump type setup, but it is usually better to go with a 20g minimum for the extra room that you would have. This is especially necessary since you want to have a refugium. Along with the fuge for a reef ready system, you are going to need a protein skimmer. Hence the necessity for the 20g.
In terms of plumbing, yes drilling is your best option. But if this isn't an option, in terms of overflow boxes CPR overflows are your best bet. Them utilized with an aqualifter pump will make sure you don't run into issues that were stated above. Along with this, plumbing is simple. The most difficult of the plumbing is putting together one made out of PVC
...and having the word difficult in this sentence is very far from the truth. It is easy. It took me ~20 minutes to put together my setup as PVC
and most of this was cutting it as I only ahd a hacksaw. I just walked into my hardware store with my return pump and overflow box. We got all the pieces necessary together, as I had measured everything the night before, and I went to work. Just make sure you get the primer and the cement for the PVC
. The primer softens the PVC
so the cement can make a more permenant bond, and since the primer is colored purple it helps you make sure you get enough of the cement all the way around. At that point, it is as simple as pushing the 2 pieces together and giving them a quarter turn.
Also, in terms of making your own sump setup, and I may be wrong about this part, but I have read issues regarding people building their own out of acrylic and them coming apart over time due to the silicon not adhering as well as it does glass. I have no personal experience with this, only passing on the information.