Like the title says, I am wondering if the the stock suspension on an R50, R53, and R56 can be interchanged. Reason being is that my sister's '03 R50 needs a new set of shocks/springs (it has over 120k miles), and I see sets from R53's and R56's all the time in the marketplace and think it would be an affordable upgrade.

Any thoughts or experience on this subject? I tried searching but nothing came up.

R53 will definitely fit on a R50. Note that the spring rates differ depending on model, but I doubt the differences are significant. The shocks should be a straight swap. Not sure about the R56, but I'd guess no.

The only differences are the front upper springs mounts, and the rear shock mount where it bolts to the trailing arm. Going from r53 rear shocks to 07+ isnt hard, as long as you get the appropriate bushing, but going from 07+ to 02-06 would be different.

Remember, other than the motor, the r50/r53 are basicly twins...there were up to 3 different shocks/struts+ jcw sold, but they all should fit.
IMO it is best to consider the gen2 mini a different car...very few parts will fit/work properly back and forth.
Rather than getting used, and at this point possibly worn shocks/struts, have youvpriced oem eqivelent's? Not premium/sport ones like mentioned, but more run of the mill ones...the auto parts places can look up the parts numbers..might be a better way to go than 5-7 struts said to br "fine" from sombody elses car....struts on the mini tend to fail gradually, in a barely noticeable way...so the fine strut might be 80% worn.....

Yea I think the spring rate differences should be small, probably not enough to tell.

What do you mean appropriate bushings?

For the front suspension, is it just a matter of swapping the R50 front upper spring/shock mounts? And for the rear, can anything be done to accommodate them seeing as how the lower shock mount is part of the shock body?

I was fairly certain that the R50 and R53 suspension is interchangeable, however I thought I really had no idea on the R56 variant.

I would tend to agree with you on the new vs used, but there is some pretty clean looking, low mileage suspension in the marketplace for reasonable prices.