The pump on the Brewtech unit is quite small compared to this unit but each to their own. If you can do it cheaper yourself and you have the time then of course its always fun to build your own.
This is an interesting one.
The Brewtech unit looks a LOT more sturdy - the plastic on the Marks unit is reported to be quite flimsy vs metal parts on Brewtech
The Brewtech unit comes with fittings to clean both inlet and outlet ports. The Marks unit can only do one at a time and you have to purchase extra parts.
The bowl on the Marks unit is small and, (especially if using foaming cleaners such as Starsan) runs the risk of overflowing. The Brewtech unit doesn't have a reservoir at all, but it IS designed for a 5 gal bucket, minimising the mess, though you do have to supply your own bucket.
Brewtech has a smaller pump which could be an issue.
And the Brewtech is designed purley for Kegs, while Marks is kegs and Carboys. I don't use Carboys so not a big one for me, but worth noting.
I especially like that the Brewtech unit attaches to the kg, so you can line up multiple buckets (wash, rinse, sanitise) and just jump the unit from one to the other. That's a LOT more efficient.
I think my personal preference would be for the more solid Brewtech Unit, possibly retrofitted with a larger pump (easy enough to do if it was necessary). The price for that unit is double the price your quoting for Marks though.
But for the added durability and convenience, I think it's a better option for me