A few topics:
It is much cheaper to do the work yourself. If you are going to keep the LR long term you really should learn to do your own wrenching.
Your drum brakes: All 109s have 2 leading brake shoes which gives better stopping in the forward direction than the common one front shoe and one trailing shoe arrangement. The downside is that without the trailing shoe the 109 brakes poorly in the rearwards direction. If you stop a 109 in a steep nose up position you will have a hard time keeping it from rolling backwards. Front disc brakes can be justified as a safety upgrade in a 109.
I have a web page that can give you an idea of what is available in the way of disc brakes for Series trucks.
You will want to convert to the brake pedal mounted power brake system. The servo unit that came on the Santana has a lot more boost that the Series LR booster and is a direct bolt on to the Series brake tower.
I have a how-to page on converting to power brakes:
The page is a draft but is fairly good.
The LR 6 - You need to keep on top of the exhaust valve adjustment. They are notorious for burning exhaust valves because they take work to adjust them.
Scotty adapters - The LR four cylinder & six cylinder bell housings take a different bolt pattern. Yes Scotty some some drilled for the 6 cylinder pattern and some for the 4 cylinder pattern. However later ones were drilled for both the 4 and 6 patterns and could be used on either.
And yes I have a web page covering these adapters too:
When you LR six becomes in need of a rebuild or replacement I strongly suggest you go with the Mercedes OM617 conversion. Robert has done a very good job designing his conversion.