Assistance with a 250 Gallon

Greetings everyone! I have a bit of a problem that I need to help fix.
I currently have a 45 gallon reef tank that is running wonderfully,
and a friend saw this and asked me to help him with his. His is 250
gallon built into the wall tank, I figured it would be a chemical
imbalance and he mentioned some pump problems. When I arrived there was
no filtration going on in the tank. I need assistance for redoing the
plumbing for the tank and pump/filter ideas. I did not realize that
this was going to be such an undertaking. So here is the breakdown.
250 Gallon Tank
1700 gap external pump – just used for water movement
20 gallon (maybe) small wet/dry filter with crappy protein skimmer
This is being powered by a 700 gph pump which pumps it through a UV filter.
He has some small power heads that we are going to remove once we get
the filtration fixed.
Lighting is ok, not a big issue 4 x 96 watts PC. More concerned on
getting water flowing.

Issues
1700 gph pump is not working correctly. Sucks air in and continually
shoots air bubbles in tank. Also the water level goes up and down in
the overflow it goes into. Will probably attach PVC pipe to this and
have water jets through the tank.
What kind of insump/next to sump Protein skimmer does anyone recommend
or not. Have about $500 to spend.

Is there a magical equation for intake plumbing and outgoing plumbing?
We are definitely up for redoing the plumbing for the entire tank with
PVC and proper plumbing materials. I have seen on some hard PVC pipes
knobs to control the flow.

Also what kind of pumps should be on the sump and the protein skimmer.

Thank you so much for your help!

David M. Lando

Wisconsin