Means the provider will deliver the circuit with an ethernet interface.
3600's are quite obsolete nowadays, try to get something more current.
What I'm trying to tell you is that 3600 series, any model, are obsolete for modern networking, and if possible you should use an ISR router, choosen according to the actual circuit speed.
Do you have a circuit of true 100mbps ?
If yes, a 3620 is too slow for that, and you need a bigger router.
If no, specify what circuit speed are you purchasing from the SP, and for which purpose.
No, you need a better understanding of networking. Routers are not like an electric plug that either fits or does not.
Every circuit speed and application requires a different router, that is why cisco makes so many models.
So unless you state what is the expected traffic and speed on your circuit, nobody can give you an answer.
Also try to understand the correct use of the rating system, it's not like you've to throw "5"s and "1"s to each answer depending how it suits you.