1: top tube is the single most important measurement for proper bicycle fit. Typically you want your knees over or slightly behind the pedals at the 3'o'clock position of your pedal stroke. When the top tube is too long or too short you will be either stretched out or cramped on the bike. You then want to get the saddle tip to handlebar length to where you feel most comfortable by adjusting stem length and height. Having a stem that is too short or too long means the bike isn't fitting you right.

Wheel base: Longer wheelbase means more stability, so you get a less twitchy ride. Great for cruisers or touring. Bikes with a short wheelbase can be found on a more aggressive frame like road bikes.

2: Roughly in the order of highest to lowest...

Dura-Ace - XTR
Ultegra - (Deore) XT - Saint*
105 - (Deore) LX - Hone*
Tiagra - Deore
Sora - Alivio
2200 - Acera, Atlus

*Saint and Hone are downhill specialized.
Alivio, Acera, Atlus, Tourney, sora and 2200 and all are entry level products. They work fine, but don't bother upgrading them since it'll be cheaper to just buy a better bike with a better groupset on it.