Initial cultivation of new veg bed...

Depends on what weeds you have. If you have a lot of the likes of couch grass then DON'T rotivate, each bit of root will produce a new plant. Don't know about the power ratings though, have never used anything like that but others may be able to advise. If you decide not to rotivate then you can do what I did on part of my plot the first year which was to cut back the weeds etc, cover with cardboard (which I tied down with pegged string) and then planted potatoes through holes in the card with a bulb planter (see below). Worked really well and the card had blocked out enough light to suppress the weeds somewhat and as I dug the soil to get the spuds out I cultivated it too. Was MUCH easier than it had been in the spring.

When you say 'grassland', are we talking rough lawn, or farmers field? Assuming that it's the former, I'd mark out beds (14' by 4' sounds about right, so you can still reach into the middle of them from the sides) and strip off the top turves from the beds, stacking them upsidedown covered where they will break down into loam.

Then roughly turn over the soil underneath picking out the big stones as you go.

Then I'd plant spuds in the whole lot (using a bulb planter) and stick on a great wodge of manure - well rotted if poss - on top of the whole lot which will break the ground up for you. As you dig the spuds up later in the year, you can pull out more big stones along with the spuds, and hey presto, your bed is fully dug, and manured.

I'm not a fan of rotivators - you'll have to get the grass off manually (which is the hard work); the rotivator won't like stones so you'll end up digging it over yourself anyway; most rotivators only tickle the top 3 or maybe 4 inches which is not doing a proper job - especially on 'virgin' ground.

On top of that, if you have any nasty perennials (bindweed, couch), you'll end up a billion times worse off once the rotivator has chopped all the roots up and they all grow again.

I'm old school on this. Dig it over properly first, either slice the turves off and stack upside down and allow them to compost down into a good loam based compost or trenchdig putting the turves face down in the bottom of each trench. This is not a large area of ground so don't be frightened of the graft.

I would plant the whole area in spuds the first year. best crop for cleaning up the ground with the constant movement of soil. Do not rotovate!!

Nah - lazybeds.

How to make a lazy bed for Growing Vegetables - YouTube

I would cover it in cardboard,chuck as much organic matter on as you can, preferably several inches, and plant straight in it. The grass under the cardboard will die and break down, as will the cardboard over about 12 months,leaving you with a lovely weed free bed that will produce some great veggies. Just top up with organic matter every year.

The easiest way is to put it under a cover for 6-12 months. After that time, you'll find the weeds dead and the worms have loosened the soil up considerably