MegaFatcat100 :
I'd really recommend building one. I'm 15 and I'm no engineer believe me!
You will really get so much more performance! My first build ever I completed today and it only took me a few hours. Now that I know more what I'm doing I could probably do in in an hour and a half. It's really not hard at all! I'd search for austin evans on Youtube. He has an amd build for 500, I wanted to use intel so i tweaked it a bit and followed his tutorial.
That being said, 70 mods??? Seesh! You don't mean all at once, do you? You can't buy a prebuilt pc in that price range. if you must find a pc in that price range that can run with a few mods and textures than maybe alienware x51? It seems to be a popular choice. I really don't recommend buying one though if you care about the money you spend and want the best preformance.
Please don't make me laugh. Austin Evans? The moron that calls office pc's that uses an APU for powering the graphics for gaming rigs, please.
LinusTechTips (link below), would be the way to go for youtube tutorials mate.
But yeah, you will be able to get alot more bang for the buck if you build the pc yourself, compared to a pre-buildt one. Anyone with an IQ of 100 (which is the normal) and a youtube guide can build an average pc, so there's no excuse for going for the pre-builds.
Only area you'd want some assistance or experience with when it commes to building pc's would be case modding, custom watercooling and manual overclocking.
https://www.youtube.com/channel/UCXuqSBlHAE6Xw-yeJA0Tunw