As long as we are asking home improvement questions...

The order in which a house loses energy is roof, floor, walls/windows.

Start with the roof and get as much as you need. Then, do the floors. If your walls aren't in need of tearing out and your windows are ok, it's probably not worth the money to replace them. If your windows don't work, are ugly, leak around the frames or are just terrible, single-pane aluminum, then replacing the windows is next.

Careful with window replacement companies and the lies they tell about efficiency. A pane of glass can only be so efficient. 2 panes and dead space is better and vinyl is a better insulator than aluminum. 3 panes and gas and dead space and even coatings can't make a window as efficient as an uninsulated wall. Also, if the problem with your "windows" is that air leaks around the outsides of the frames, you should be able to fix that without replacing them if they're old, good looking wood windows it's a good idea. Storm windows are awesome too.

Spray foam is overrated as far as insulation goes. It's a vapor retarding insulation so if you're only doing pieces of the house, you're over paying for R value as opposed to batts/blown in bulk insulation.

If you're doing a bathroom at a time or a bedroom, just use the batt. Don't waste your money on foam for only a part of the walls. That said, if your house is off grade, old and leaks like a sieve through the floor like mine and most, then I would definitely consider 3" of spray foam on the underside of the floor decking. Of course, make sure all electrical, ducts, framing repairs or whatever are done before or you'll be cutting it out to fix anything. In some cases, it's a good idea to put down a heavy vapor barrier on the ground in an off grade house and then top it with gravel then spray foam only the foundation walls from the inside of the crawl space. Less foam square footage is less money and the natural leaking of the house will condition the space maintaining a dry and climate controlled space.

I love the space heaters in my old bathrooms and had to argue with the carrier to keep them. They're on 10 minutes each 4 or 5 times per year. Don't see the issue, but still, your best bet for efficiency is high seer hvac units or geothermal and insulation in the most efficient order.

Call the local utility and see if they'll come do a free energy audit and call your cpa who should be able to direct you to tax savings on the retrofits.