Sectional garage door is too narrow.

First job - scored a sectional double garage door from gumtree.
Problem - my opening is approx 5.5m wide - the door is only 5m wide.
That means that I have to panel about 250mm on each side of the door.
Concrete poured slab and drive, wooden crossbeam (plenty strong enough - it is huge), and steel 150x150 uprights.

I need new uprights to attach the door rails to - what is big and strong enough, without being overkill and therefore more exxy than needed?

What size for the bit do I bolt into the concrete at the bottom to join the upright to? How do I seal that against the concrete?
The water will run away from the front of the garage, but I will need to seal it so it doesn't get wet and rot.


The work is relatively simple stuff, I just have no idea on sizes, or for that matter the best bottom strip to put on.

I have some ideas on this stuff, but I am sure someone here can just tell me the best way. ;)

Thanks,
Dave


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