Which engine type would you get and why?

I'm not an engine guy at all.

So.. with that I come to you to ask for advice.

Still looking at possibly pulling the trigger on a 2100z.
It will have a 52 inch deck (I need to get around some spots)

So my engine types are:

Kawasaki FX801V
Vanguard 810
Vanguard 810 EFI.

Love to know your reasons for your pick

https://www.vanguardengines.com/na/en_us/home.html

Either Vanguard engine, you take care of engine maintenance wise & it will give you many years of service. Tough, durable, professional grade engine that will not let you down as long as you keep it maintained as per engine manual maintenance intervals. I do check engine oil level & air filter ( when dusty conditions) before each use, currently 387 hrs. Also valve adjustment is most important at specified intervals in engine manual. Always use B&S parts.

The plain Vanguard as the EFI will have all sorts of electronic controls which
1) I will not have the correct equipment to diagnose or repair
2) the electronics will be specific to vanguard so when they want to randfather the engine, they just stop making the electronics
3) electronics on mowers tend to give more trouble than similar stuff on cars.
4) you will be restricted as to where you can send it for service so if the local Vanguard person is not up to scratch, you have problems.