Its really up to you what you want to do.
Leafs can be cheap and easy. If you do a spring over it is so easy to cut corners and end up with a crappy setup. Just like anything else, if you do it right it'll perform well. Chevy flat springs are commonly used and will perform very well. You'll probably need a rear traction bar and definitely extend your brake lines. Steering will also need to be addressed. A longer drag link and dropped pitman arm are needed to prevent severe bumpsteer.
If you cut corners here, it'll cost you in the long run. I've seen people use crappy leaf springs and within a month those springs are junk. No good. Also short brake lines easily rip out. No traction bar will wear out your passenger rear spring in a month, causing that corner of the vehicle to sag.
The main reason I say stick with the current Rover frame is because then you don't have to figure out how to weld all the tabs for engine mounts, body mounts, etc on it. They are already there! The rover frame really does lend itself to a link suspension coversion very easily.
Also using links will allow you to retain an excellent approach and departure angle. Having leaf hangers out front can be a pain.
Linked suspensions are going to be more expensive. I'll tell you that right now. But they will perform better and ride better than leafs. I see it all the time. I say go 4 link rear because it is strong and moves freely. A 3 link front will allow it to move freely and the track bar, if the proper length will prevent any bumpsteer from occurring. 3 link front and 4 link rear is what I see most of the time and it works.
Whatever you decide to do, do LOTS of homework first. Going out and doing a suspension is not easy nor a quick job. Having people around who know what they are doing will make it go smoother as well.
btw it rolls and has 3rd members ect
