here's what I've used for over 3 years without a problem,and I'm sure you'll not beat the price
a kent float valve...about $20
RO/DI bulkhead,any good on-line fish store that sells RO/DI filters will likely have these,got mine from Marine Depot...less than $5...it doesn't come with the rubber gasket,but Wal-Mart has a repair faucet kits that had a gasket to fit it...
5g bucket,couple dollars at Lowes...free after you empty a salt bucket
I added a shut-off valve for kicks to mine but it's really not necessary....less than $5
and the RO/DI tubing is cheap too,like $0.20 a foot,mine was left over from my RO/DI filter install....I have since replaced the 5g bucket with a bigger container,I got tired of refilling the bucket every day
the bucket is higher than the sump so it will gravity feed...I have had my RO/DI filter connected directly to the float valve but that is a disaster waiting to happen