I decided to do what your thinking about (coloured grout on floor, then white grout on walls). I think it looks OK not odd, but obviously is personal opinion. Pic attached part way through work, I didn't use a grey marble but a gold vein marble. I think I would have preferred tiles lined up between floor and wall still but you don't notice too much after a bit. Gives you an idea on effect

I don't think the weight will be an issue as long as it is not a very finely supported island. If you require castors then make sure that the legs are straight not turned to a bun foot like your image. You need as much leg as possible to fix the castors. If you go for a 20mm thick marble or slate top you will not have a problem with weight or moving it. Get the castor type with brakes so that you can lock the position once you have the right spot for it.

I had a friend with a real marble table……. It had so many layers of protective seals that it no longer looked like marble. But there are alternatives that are better than marble but with the right look. Personally I like ceramic/ porcelain tables. Italians make some of the best porcelain. Although a shade more expensive than plain marble, porcelains have the pattern printed on them meaning you can get a table that realistically looks like a very expensive heavily veined marble without the price tag of the real thing. Furthermore they have great stain and scratch resistance.

If you want to strip the paint off, have a look at a company called Strippers paint removers. They have a very good website, I have used two of their products to strip paint off different surfaces with really good results. It is a bit of a messy process and may take a few days but it will be worth it.