Considering buy a CZ 550 rifle..

What should I do to it to make it a first rate hunting rifle? Not based in US, so AHR is out.

Maybe a 3 tier list?

1. Make sure it feeds and ejects. Slick up different parts of internal action and bolt? Which?
2. Change trigger, safety, barrel bands
3. New stock, bluing, checkering.

First you have to get it.

Use it.

See what you want to improve and do it.

Nothing. They are good as is. I had a 375. Needed nothing but shooting.
My take on some of this customizing that goes on (and don't get me wrong, personalization is fine); given a decent quality rifle to start with, and most are, training and shooting under field conditions are far more important than the hardware. I see, all the time, guys at the range with new, and sometimes expensive customs, who can't shoot, and don't want to learn. Self taught, they think they know everything. Fortunately, the PH will compensate for this lack of skill.
Practice with it. Oh yeah, have to guy it first.

Definitely make sure it feeds/functions. The trigger is good as is. I don't like the safety. I like barrel bands. The stocks are incredibly clumsy and can be slimmed down quite a bit. There is often nice wood under the gorilla glue they use.

What caliber?

In my opinion, like all factory rifles, the CZ 550 is a starting point. But the CZ, unlike some, is a superb starting point.

I like a single stage trigger positioned at the rear of the trigger guard, a three position safety, a barrel mounted front sling swivel and a solid and straight bolt handle, at a minimum.

But none of those modifications is essential; all are just personal preferences.