Mercedes G wagon help

[Jacob.m]

Hello all

So iv been searching for a long time and have come across a G wagon i finally like with a great price,

the problem i have is the fact its a 1989, and needs a fair amount of work but not loads.
its MOT'd and runs ive drove it ect..
my only concern is the engine, it drives okay..but its its very rusty under the hood, is it worth engine swapping it with a 2001 cdi engine or something? or just cleaning it up a bit and adding parts.
its a 3L Straight 6 auto but as its old it has this weird "auto box" its got Gear 1 Gear 2 Gear 3 and Gear 4, you select these, and then drive in it you cant change it whilst moving it does not change automatically...defeats the object of auto i know... but theirs no clutch.
ultimately you stuck in one gear.

does anyone know any ways around this? i love the car just this idea has me thinking am i going to be redlining it everywhere stuck in 4th at 60mph???

thankyou

any feedback appreciated.

[Bodo]

Put it in 'D' for driving forwards, and 'R' for driving backwards. Put it in P when you park the car. Only use the numbers when you want to limit top gear.

[Jacob.m]

Appreciate the reply, but the reason why im hesitant is because it doesnt have D, its almost semi auto where its such an old car, it has N, R, then its goes
4, 3 ,2, 1

and thats it...