We had a Harmar lift for our SUV - only used it for 4 months and my PALS has fairly slow progression. She became unable to make the transfer safely - so she rides in her chair in the van.
Other problems with carrying your chair outside the vehicle: chair is not supposed to get wet - maybe you can secure it against light rain, but anything more is a big problem, especially being splashed by a truck or going over a deep puddle. Battery is not supposed to freeze - thus leaving out a lot of winter. Another problem is that it changes the stability of your car. There is nothing you can do to keep it from bouncing, at least a little, and the load is in the worst possible place for safety - high and in the rear.
We really should have gone straight to the van.
Also - buying the Harmar lift lulls you into putting off the van much longer than you should. With ALS I learned to think ahead - buy the ultimate thing you will need - wheelchair, lifts in the house, bed, bathroom remodel - much before you think you need it because you will need it sooner than you think!
P.S. There will be no way the external chair carrier will be all that you will need - save the money and go straight for the van - you will eventually need it or be stranded.