Should I have a go?  

Cutting foam isn't that hard. I used to use an electric bread knife but switched to just a non-electric serrated bread knife years ago. I mark the foam with a Sharpie, then just draw the knife blade along the mark with enough pressure to cut but not compress the foam much. If you compress it, you'll get a distorted cut. You have to be very careful to keep the blade perpendicular to the foam.
Some big upholstery shops and big foam places have foam saws--those are the best. But they are $800+ so kind of out of reach for DIYers!
But that's the easy part. Are you sure you need to replace the foam? The seats look like they really need it, for comfort's sake if nothing else---but the backs?
I see several problems. First, it looks like there's a seam across each back cushion. If both sections have zippered openings, then ok---but I'm guessing they don't. Cushions like these are made by putting two foam blocks in, then stitching through the middle with a big, very specialized industrial sewing machine.
Even if it was possible to replace both parts easily, you will lose that squashy look that is part of the appeal of vintage leather furniture like this. Unless, of course, you can figure out the exact combination of density of foam and thickness of batting wrap to get the squashy look but with better support. I would find that very daunting and I've done a lot of re-upholstering.
On the seat cushions---looks like they have a rounded front edge. I can't tell if there are seams across the cushions--probably not? If it's just one big piece of foam and not two pieces, then it's more do-able than the backs. But you should still do a fairly substantial wrap of good quality upholstery batting (NOT quilt batting---you will have to find an upholstery supply store locally or online because stores like Joann Fabrics do not sell upholstery batting). If you just put only foam in the seat cushions, they will have a very well-defined shape that will make the back cushions look sloppy and tired instead of comfortable.
There are two ways to get a rounded edge. Either sculpt it with sharp, long scissors or a razor, or cut a V groove into the front of the block with a razor and spray both sides of the V liberally with spray adhesive and press together. This is pretty hard to do well on thicker foam but will yield smoother curves than sculpting.
Use something like 3M Heavy Duty Spray Adhesive, not a cheaper brand because they do not hold well at all. You will need it also to attach batting to foam. If you don't glue the batting down, it will creep over time and end up all bunched inside.
Have I scared you off yet? This is DIY-able if you're willing to hunt down the right materials and are can afford to maybe ruin some foam in learning to shape it just right. It might be better to leave it to the new owners to deal with, or to see if you can get a pro to do it correctly for a price that won't cut into your profit too much.
Good luck...

Thanks spanky for that great in depth reply!
I shall take on board what you say and decide over the next few days whether to go ahead.
The 'back cushions' are two separate pieces of foam which as you say will make it a bit easier. I live in the UK but should be able to get all the materials etc if I decide to take the gamble........!