re: how much to replace a 5 ton ac unit?

You mean the old air handler inside won't work with a new condenser outside. This is correct because the old parts will be for R22 refrigerant and new will be 410a. You can put a different valve in to change the condenser to work with the old air handler being R22, but they are phasing out all R22 and it is hard to get parts and the R22 refrigerant is getting expensive and ac guys want to charge you a ton for it. Just change the whole set up and be done, save yourself headaches down the road. Don't worry about cleaningnducts or anything with them. Drain pans don't cost anything and you don't need a pad the new unit will go on the same pad the old one was on. Unless you just want to write a check and be done your best option is to get someone to order your materials for you at cost from acme, cob urns etc and pay a A/C tech to come do on the weekend. You will save yourself a few thousand