The standard type bridgeport knee style mill, has been around for a long time, and it's hard to tell from a distance from one thats new or old, as the overall shapes of the castings are very close, if not identical
The J head is an older model of head that has step pulleys, to adjust the spindle speed, manually by moving the belt, it was a fairly reliable head, but with step pulleys you do not have infinate speed adjustment, as with a variable speed head.
Almost any head can be fixed, there are plenty of parts available for bridgeports, so you really need to check the stuff that's hard and more expensive to fix which are
1-wear on the quill,
2-wear of table ways,
Normally table slides will wear in the center, as most work is done in the center, so the table will become loose in the center and stiffer as the table is moved to the ends.
Not as comman but another good make of knee mills to look at is excello
hope that helps a bit, and hope someone else can put in their 2 cents too