Not sure how you are going, used or new...but either way, it is an investment.
The DF520 is doing well in my house with 100's of ft of cast iron baseboard and tons (literally) of cast iron stand ups.
3 kids and laundry does not even phase the 520 and furthermore, cold/windy weather it chugs along happy.
One thing is if you even add onto the heating load with addition and/or garage, you are gonna need some firepower.
I dont have the experience with garages, but from what I read, they have a serious load.
If you are putting out cash for a stoker, better be safe than sorry.
Upgrading AGAIN in the near future will set you back and will be heartache if you have to postpone until it is able.
Explore you options and make the final heat loss your goal in determining your needs.