1) not sure, but probably 128 (can be hacked somehow to 255), 1.1 will officially have 250 AFAIR
2) there is element limit not vehicle/whatever limit. 65535. shared between objects, vehicles, peds, markers, etc etc. afair - there can be up to 254 streamed in elements (Polish wikipedia says that MTA 1.1 will have no limits for elements - but i'm not sure if this is true thing). samp have 2000 vehicles, right? 2048 pickups, 100blips, etc?
3) read "2"
4) depends of running resources of course. empty server with nothing but just spawn will send only player movement data etc, but resources - more data to send.
if you want to choose between samp and mta - choose mta - much better sync, MUCH MUCH better scripting possibilities, easier scripting, serial assigning (making every player have unique "ID", so you can auto-login them etc)