For the usage you have described I am partial to Ford’s Taunus V4 engine. The engine is a 60° V4 with one high speed balance shaft. Initially introduced by Ford in Germany in 1962, the engine’s arrival on the automotive performance scene is pretty much picture perfect for your build.
As luck would have it, Ford built the German V4 in their Cologne plant and used it to power the Ford Taunus and German versions of the ever popular Consul, Capri and Transit.
This is a crankcase side view of this stunning powerplant. Be sure to check out the amazing high speed balance shaft to offset the NVH of the inherently and notoriously out of balance V4 design.
Unlike longer ‘V’ engines with more cylinders, the connecting rods do not share a crankpin on the crankshaft.
This V4 was later expanded into the Ford Cologne engine that was used in Ford’s Capri, Taurus, Cortina, Consul Granada, Sierra, Scorpio, Ranger, Explorer Mustang and Mercury Capri and many other cars. The V4 engine also has a robust history of use in industrial applications, and agricultural machinery not to mention snowcats.
Initially the engine was designed by Ford for a new entry compact car intended for the US market to be called the Ford "Cardinal", which eventually evolved into the Taunus. Ford changed their mind abandoning the "Cardinal" project and instead building the Ford Falcon for North America.
Ford substantially expanded the already impressive application range of the V4 engine which was also evaluated in the ever popular Saab 96. Ford bought several Saab 96s for testing and eventually sold the cars back to Saab with the V4 engines still in them. Saab tested the V4s at their Trollhättan test facility and was so impressed they decided to acquire the V4 engine for their 95, 96, and 97 models. The V4 engine provided massive low end torque. Saab dealers offered first owners a "Lifetime Warranty" for only US$50!
I mean just how much better can it get? This is obviously the engine that was made to order for your build — and, will meet or exceed all your requirements. Of course, let’s not forget the prodigiously wide power band this engine was available in, going from an entry level 30 kW to nearly 200kW!
This engine has your build written all over it ...
Ed