Well a computer can never have enough power. With an i5 and 8GB RAM you can model quite a lot. The i5 is sufficient in my opinion. In general, the faster the CPU the faster you have your solution. 8GB RAM is okay but 16GB would be surely nicer. Especially if you plan to run larger models with many finite elements. A quad core processor still has some performance left while running serial simulations (not parallel). So you should make sure that your RAM also has free reserves, because this way you can continue working during the simulations. This could be tight with 8 Gigabyte. But you can certainly start with that.
If the models are getting bigger, it might also make sense to look for an access to a computing cluster.
I hope this helps.