Hi, peeper
See if you can spare the $300-$500 for a real cake. Maybe sacrifice something else? Like flowers or an extra photo?
I paid $4XX+++ for mine, offset $30+++ in exchange for the small one the hotel would have sent to our room as part of the wedding dinner package.
As for the dessert buffet, maybe can just choose those different from your wedding cake. E.g. if you are having fruitcake or chocolate cake, then for the buffet choose less chocolate stuff like chocolate mousse or another chocolate cake. Maybe go for mango crepes or pudding, mixed fruit tarts, soupy types (maybe hot for the older guests) and just one other type of cake, maybe cheesecake.