Which model would you recommend?
I would recommend the Woden W78 or Record No. 778
Can you just use a plough plane?
You can indeed use a plough plane to cut certain rebates. A dedicated rebate plane has various advantages. It has a greater width of cut, as well as being able to use it with the fence off to clean up wide tenons and in other various situations. It depends on whether you regularly come across situations where you could have done with one.
What do I need to do to prepare the plane for use?
You will want to check the flatness of the base and possibly flatten. There is no need to work the sides at all, use as is.
No need to feather edge the corners of the plane iron, keep them straight and square to the edge.
Fences and depth shoes
It is probably not necessary to flatten the fences and depth shoes. You can feather the leading edges with a file and soften with abrasive paper to lower risk of jarring.
How do you set those without adjusters
To set the iron, loosen the lever cap tightening screw near the upper end of the cap iron just a partial amount, until you can tweak the iron to set it visually and try to gauge the depth so that the cutting iron protrudes the correct amount. Further depth can be tapped even with the cap iron tightened.
This is quick and effective and not inferior to those with adjusters once you are used to it. Definitely no retrofitting with adjuster necessary. It’s not worth the effort.