You should be able to check an Apple serial number online, for when they (Apple) think it's first year warranty runs out. However I guess you won't get a serial number until you get a machine. If it is a same spec machine as found on the Apple website, I can't see you'd go far wrong.
Bear in mind that a new machine will be running Catalina, and you probably cannot roll back to a previous OSX. Which means that 32bit software won't work. I'd check the software that you use regularly, or the is mission critical, to make sure it will run in Catalina.