@GloomyCallum Among the options you listed, the iBuyPower is clearly the best option of the three. I would never suggest anyone buy a Mac for playing Sims 3, let alone one with hardware that wasn't particularly strong even in 2009. The game doesn't run well at all in macOS, but even aside from that, you could do a lot better.
The third option would be fine, but the second has a stronger processor and a significantly faster graphics card, so given the equal price, the second one is better. The third does have more memory, but RAM is the easiest and cheapest (under $50) component to upgrade of all, if you decide you want more than 8 GB. Sims 3 won't need the extra RAM anyway.
If you'd like other options though, here are a couple. This one is $50 cheaper than, and has the same hardware as, the iBuyPower, except it has a much faster hard drive, which would make Sims 3 load faster in everything from the initial load screens to CAS and Edit Town to travel transitions. 500 GB is also more than enough storage for most people.
This one is used but sold by Amazon Warehouse (for $633) and has a stronger processor than the one above, and an equivalent graphics card. It also has a 1 TB SSD, in case you really want all that extra storage.
I didn't see anything better than these for $650 or less, and it sounds like you'd like to keep the price down in that range. If you want to know what's out there for a slightly higher price, let me know. And please feel free to ask more questions.