+1 I have a Mark I in 357 and I love it. I've even started to reload 357 just so I can shoot it more. It will not cycle 38s at all and as EasyD indicated, you need hotter loads to cycle the action properly. The minimum it will eat properly is the 125gr rounds, but it prefers the 158gr rnds. The 125s are fine, but you will get a failure to eject properly every once in a while.
As to the conversion, it will depend on which model you get. The Mark Is will only convert to 44. You cannot convert to 50ae with a Mark I. The later models you can convert between the three. Also as EasyD indicated, the conversion is expensive. If you go to Mangnum Research's web site you can see the conversion prices. Not cheap at all. Not to mention that DE in Mass will cost you easily over $1000.