I am a fairly recent returnee to TS3. When I picked up the game again, back at the end of September (or was it October?) I hadn't touched it since mid 2012. However, I had a hankering to play one of my favourite families again and recreating them in TS4 just didn't cut it. I am a Mac player but I did play on my Win laptop while waiting for the new Mac version of TS3 to be released.
Technically you will probably have to do a bit of reading about your video card and how to get the game to recognise it. I had to do that on the laptop. It's not hard to do so don't be put off. There's a lot of information and expert help on this site and people with knowledge who can help you if you need it.
I had no problem downloading my store bought items. I downloaded them on both the laptop and my Mac.
I am loving TS3 again. It may take longer to load and save and it is not always as slick and smooth as TS4 but it has a lot of depth to the gameplay and the sims themselves are so interesting. It is not just the different art style; the sims' personalities and the forces that drive them are so different and can be challenging. It is easy to have a bunch of happy, friendly sims in TS4. My TS3 sims are a lot more grumpy and quirky but a heap of fun.
So, my advice, is to go for it.