Several folks run the stock 45mm Offset wheels on all 3 wheels (or 4 in some cases), so I wouldn't think you'd experience any problems. Just keep in mind that the narrower wheels may result in some loss of traction. www.willtheyfit.com will generate a visualization of how the new wheel size will sit compared to the stock wheels and will also show how a larger or smaller diameter tire might affect fitment.
I'm assuming you want to run the same size all around so you could rotate them? You'd be better off running staggered like the factory. A wider tire in the back has several handling and traction benefits. As far as cost savings go, unless you go with something crazy size, the cost increase is minimal and worth it.
Another thing to keep in mind is will the tire you want fit the wheel size you want. Check the tire manufacturer's website which should usually show the minimum, recommended and maximum wheel widths recommended for a specific tire size. For example, Nitto says their 555 G2 tire in the stock Slingshot SL rear size (255/35R20), while recommended for a 9.0" wide wheel, will fit wheel sizes between 8.5" - 10.0". See - NT555 G2 | Nitto Tire.