Welcome to the forum.

Larger is better for your 1st humidor, you will out grow a smaller one fast.
Yes you can use Jars with the lid on to take up space when you have less cigars in the humidor.
I would avoid using things like wood "solid blocks" unless it was something like mahogany or Spanish ceder.
For hygrometers look at a good digital one the analog ones look nice but suck at accuracy.

Also for ordering a large amount of cigars at one time your better off getting a few to see if you like them them order more.
Your taste will change over time so that milder cigar you loved last month/year may be to mild for you just a few moths later and your stuck with a bunch of cigars you no longer enjoy smoking.

Its not a good idea to spend $1000 on cigars at one time unless you know your going to like them.