If you want to learn more about overflows and such, I did a write-up on another site ... this is it
I recently sold my Freshwater gear and started a long research project about overflows as I am going Saltwater. I learned a lot over the last couple of months, so I hope you read my entire post and it's included links
First, you shouldn't try to completely replace your in-tank flow with just a return pump. You may end up with low-flow areas or dead-spots you don't want or didn't plan on.
You could get very involved with a manifold return, with many outputs pointing every which way, but you are also then relying on just one pump for the health of your charges.
The 'U' style overflows work, but rely on a 'lifter' pump to restart when the return pump is powered back on. Relying on the pump to restart when, say, you aren't home, could starve your fishies and beneficial bacteria of oxygen.
You also have to consider the risk of your overflow becoming slowed or blocked and your return pump draining your sump while it is subsequently overflowing your DT
There are DIY PVC pipe setups
My DIY PVC OVERFLOW TUBE
And there are DIY overflow boxes
DIY Overflow Box
Now, about drilling and so on...
Be very careful where you drill your tank as many, or even most, tanks have tempered bottoms and you will destroy it as it can't be drilled. Drilling the back is a great option. I have included for you some links so you can explore your options. I suggest you take your time and absorb all 3 options I linked as they all have their benefits and drawbacks, such as;
noise, placement, tank-to-wall clearance, flooding, and cost.
This first link is cheap and quick
Complete Overflow Kits - Glass-Holes.com dope aquarium stuff
This second link is to what's known as the 'herbie method', and I have provided for you the designer's original post. You can find compressed explanations if you search the web for Herbie Overflow
This method is nearest to fail-safe
NEW plumbing method for an ULTRA QUIET REEF TANK!!!! LONG !!!! - Reef Central Online Community
And the fail-safe, no flooding ever method...barring your overflow box doesn't leak
BeanAnimal's Bar and Grill - Silent and Fail-Safe Overflow System
Once you thoroughly read through these, and others you find on the web you should have a better idea what you really want to do.
Take your time planning this as it's a big decision.
Good luck,
Greg