Save as much money as you can! Paint, fabric, lamps and accessories are inexpensive fixes. If your lease prevents painting, purchase stock drapery panels that you can take with you (54x84 works everywhere!), add a colorful rug and plenty of lamps. Estate and garage sales, consignment shops and friends and family can all be sources of great pieces. Add things as you find them- and it's cheap entertainment! :-)

Good point on the stock panels. I was going to avoid window treatments but standard panels (preferably on sale) could be great.

We plan to paint so I'm considering playing with some colors that I might otherwise be too nervous to use.

We need a few sofas and I don't want to buy "disposable" furniture. Can I spend on nice sofas that can easily be worked into the new house?

I might spend on mirrors that can be used in guest rooms and as accent pieces down the road (never need to throw out mirrors, right?).

Would re-staining wood floors be a complete waste?

Don't put money into the house, only into furniture and decor you will take with you.

I'll suggest just the opposite of getting a bunch of stuff that may not work in your next place. Purchase as little as possible--and save money. First try to identify what is your decor style--books, magazines, online info such as Houzz series on different styles. You don't have to lock in on one in particular style--just note the elements and the kind of furnishings and colors that make you happy and comfortable.

Then slowly start to purchase pieces that have multiple uses--can fit in a living room, a bedroom or even a family room. For example, a chest of drawers with good bone structure, design and interest. It can fit in a dining room, a bedroom, family, room, even a hall should you purchase a house that has a large entryway.

Even an over-sized love seat can fit into a large bedroom, a living room or a family room. Mine has lived in a family room and three bedrooms and a guest room. I've had it for 20 years, added a custom made slipcover and still love it. Just focus on style that will not be dated--so be careful about getting trendy items. Estate and garage sales, etc are good places to shop. But focus on pieces that have multiple "possibilities" and if need be can be refinished, recovered, etc.

Use your dining room or living room wall as fun planning place for your next dream location!

Good luck!