I got my little pump from Brewbuilder for £47.50 plus the cost of a PSU. It handles some solids, and you control the flow with a tape on the output. Flow is pretty decent with them (TS5-mini mag) and the 1/2" BSP fittings on them are stainless steel.
Buffers, you are massively overthinking/over-engineering a simple re-circulation setup for an ACE boiler, massively. You seriously don't need all of that. Some silicone hose, some QD/camlock fittings, a pump and a tap to regulate the flow (you don't want to run full flow for re-circulating overly). With my little TS5 I didn't need to fasten it to anything, I just attached it using some 90 degree elbows and QD fittings to the tap on my ACE, then the hose to the other end. Cut a hole in the lit of my ACE, fitted a bulkhead and a T piece on the inside for the return from the re-circh, job was done. Lag the hose with some pipe lagging, and you'll have even less lost heat when mashing. I even made a fitting that I could hook up to my hose to do a bit of a whirlpool when cooling the wort with an immersion chiller.
Oh and yes Foxy, I used to get around 80% efficiency with full volume BIAB, no sparging. Mash for a bit longer, make sure when you "stir" you actually lift the malt off the bottom rather than just swirling the liquid, dough in thoroughly, check you have a decent mash pH, all of this helps efficiency more than re-circulation. In fact, the main reason I added re-ciruclation, well it helps if you want to have a go at doing stepped mashing, it also helps with getting clearer wort, until you squeeze the bag.... lol A gentle squeeze you're ok, too hard and you squeeze all the flour out of the grain right into your wort.... I used to try to tie my bag really tightly over the top of the bag instead, then tighten it more and more, with the bag hanging over the boiler. If you are using the ACE bag though, it's not designed to be squeezed, all you can do is find a way to hang it over the boiler whilst you wait for the wort to come to the boil. Here's the thing, the more you liquid you get out of the bag and into the boil, the higher your efficiency goes as your volume goes up. Get a larger volume of wort of the right SG, boil it enough to get to target OG, you have a larger volume of wort at target OG, et voila, higher brewhouse efficiency. The more you lose along the way, the more your brewhouse drops off. Another trick, bag your hops in the boil, skim the foam when it reaches boiling, then transfer EVERYTHING out of the boiler to your FV after cooling, again, more volume in your FV.... lol
