Basically, apart from all the usual.....reliable, easy to....etc, etc, etc
You need a reg that will look good and not put the punters off after they've been round the block for several years.
Ist stages generally hold up well, but it's the 2nd stage that the customer sees just before they put it in their mouth that needs to look good.
You have a choice of Sherwood, Scubapro and Mares.
Let's cross off Mares as their 2nd stages don't hold up and pass the cosmetics test very well.
Sherwood....the old oval shaped 2nd stages (even though they had a soft purge cover) held up pretty well. But there's a new model and your tech has doubts about it.
Scubapro....thankfully, they've done away with the R195, 290, 390, 295, 395 series of 2nd stages and have a new G290. Historically Scubapro's have held up well in rental and the G260 looks to be better than what it replaces. The main downside are the hoses that can't be undone with an adjustable spanner.
Either the Brut/Magnum or the MK2 G260 would be great choices.
Which rep do you like the most?
Your "keymen" will prefer Scubapro.
Who does your BC's?