Is it cheaper to buy a furnace and AC together? (Should I replace my furnace and air conditioner at the same time?)

Remember to factor labor into the budget. Many times the labor can cost as much as the equipment. Therefore, spending a bit more now to replace everything can be less expensive than re-using an older furnace that still works, or putting in a furnace with a plan to ‘add air conditioning later.” By replacing everything at the same time, you will get matching equipment that is all warrantied together. Then you can typically not have to worry about the system for another 15 to 25 years with proper maintenance.