I would say build the PC if your'e going to use it a good bit, like playing games, editing and that. Also make it powerful as well, I have no idea how much that engine would cost but I'll guess around 300 for the max, in which case that is the best. If you really want to build a PC you should go all out making a fancy thing, like a water cooled one or put it in a desk.
if you aren't doing something like that definitely get the engine it would be a far better experience, again I have no clue what this budget is.
I'm edging you to the engine just if it's not clear.