I think the standing of the bride affects how much prestige you get out of the marriage: the daughters or sisters of prestigious kings can get you quite a lot of prestige, whereas a random courtier will give a bit, but not enough for it to be a criteria you need to consider.
If you're marrying off other dynasty members, you get a bit of prestige too, and I think this amount is inversely proportional to how closely you're related: your children's marriages give you less than your own, your cousins' marriages even less.
I mostly look for a culture I'm happy with (for my Welsh game, I've only married my stepdaughter to a Norman, my dynasty mostly with various Celtish people, with some French and Occitan thrown in), young/about the same age, stats, and if I have the chance and especially if I'm looking for a bride/groom for myself or my direct descendants, then I try to find someone as important as possible, or actually related to someone important.
The inheritance game is usually quite hard to play, it's a lot of work trying to find heiresses (in realms with semisalic primogeniture most preferably), and you usually have to do it while they're underage, then save up a dynasty male to marry to them the moment they turn 16 (and keep watching her for this!), and even then, the proposal will very possible be declined. Would be nice if the ledger had a column stating if the potential bride's children would be eligible (say, within the first 5 or 10 people in the order of succession) for an inheritance.
It's either that, or you assassinate the heiress's/female rulers husband before they have male kids, and marry them. Although female rulers practically never accept proposals. Assassination generally makes inheritances easier.
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