6.52 will work, after a fashion, in OS 10.5 but you're right, it's not entirely compatible. 7.31 is and yes, it costs to upgrade, have a look at the Quarkstore link from the Products menu at the top of this page.
There are lots of places you can buy fonts online - I favour faces.co.uk - but do a Google search for font providers and you'll find loads, prices vary a little. I'd also recommend, if you don't have one, a proper font management utility - Apple's Font Book is rubbish and not meant for 'professional' use. Linotype's Font Explorer is good and free, other options are Font Agent Pro, Suitcase, etc.
Good luck,
Greg