Macbook Network Security Puzzle (Help Please!)

Hi, Thanks for taking time to help me with this security problem that I've been recently experiencing with my Macbook internet network setup. To put it simply, I am concerned that my apartment neighbor is a hacker that is trying to spy on me just to irk me and disrupt my peice of mind! (I found out through Facebook that he is a network engineer at an office) I will spare you the details of why I think this, so I'll just ask a few questions and hope that someone can help me (and potentially others in similiar situations) gain some peice of mind for happy internet sailing!

If someone can help me answer these questions please:

// Is it possible for the neighbor-hacker to spy on my internet activity, or somehow access my computer remotely? (a Macbook connected directly to a broadband modem, no wireless internet is involved)

-- I am concerned that he might have gained physical access to my room and saw the numbers and letters on the actual broadband cable modem (is this enough information for a hacker to somehow get my IP address and spy on my internet activity?) There was a password on the computer, so I'm not concerned that he actually uploaded spyware to the device. Or could he somehow hack my ISP to get my activity.

There was a window of time when the firewall feature for the macbook was not activated (it's now activated and a fresh OS installed, with file sharing, screen sharing, etc. disabled.)

// Would it be possible for a hacker to directly attack someone they knew if they were motivated enough to want to?

I don't know much about network security so my mind is maybe running wild with possiblities, but still I can't stop thinking about this. If someone can please help me it would be so helpful and appreciated!! And if my network is unsecure would buying a router help? Or installing software firewall protection? I am just concerned about internet privacy, I feel very not at ease if I think that my neighbor somehow knows what I'm doing on the internet. Thanks very much for your help!