HVAC replacement/swap out. Cost for labor? Doable by myself?

[Jeffrey Douglas]

So, I just got an offer accepted on my first home, super excited.  Anywho, this crappy sellers market kinda forced me to go the fixer/pretty-upper route. The home has a probably 10-15 yr old HVAC system, obviously I'm gonna get an inspection and see what they say and go from there. But, was just browsing online and stumbled upon a guy selling a decent trane 3 ton unit close to me that he apparently bought for a home he lived in temporarily or something and only used for 1.5-2 years.  Anywho, as odd as that sounds the unit looks great, comes with everything and he is only asking 500 bux OBO.  Apparently he's just sick of it taking up space.  This got me to thinking, with ductwork/wiring/etc already done is simply swapping out/upgrading the unit doable by a moderately handy person such as myself?  My dad is an electrical contractor, so could use his help for semantics, but my main ? is do I have to have a license to do this?  I'm assuming probably so, which leads me to my next question.  If I have the unit and all the stuff needed, how much will an HVAC guy charge me to simply pull out the old unit and put my "new" one in and get it set up?

Thanks so much in advance,

Jeff