I use Plusspec on three machines; one is a $400 special it was the cheapest computer that I could find in store and PlusSpec seems to work okay... Yes, the graphics are not as good, and that's due to the fact it does not have a dedicated graphics card, yet, other than that, provided I do not have too many applications open at once it works as good as can be expected.
However, my desktop is an animal and man you can get through a lot more with a higher spec machine with minimum eight gigs of ram, solid state drive, and a dedicated graphics card.
My tip is if you are using PlusSpec every day, get a big computer as time is money, however, if you only use it occasionally then there is no need to spend so much money.
In the interim buy what you can afford if you have some spare cash use it on training, the extra money you make via the use of PlusSpec can be towards a faster machine.
No matter what your productivity will be ten times faster than using Sketchup alone, Archicad or Revit even on a low spec machine. I do recommend the biggest screen/monitor you can get.
I hope it helps