upgrading my 2009 macbook pro 15"

I love my MacBook Pro 15" laptop. Purchased new in 2009. Alas, I fear it is coming to the end of it's very productive and reliable life.

I am looking at the McBook Pro 13" Retina as a replacement.

On line, I see I can get with 2.7 gh or 2.9 gh dual core or 3.1 ghx dual core.

What is the difference?

I also see I can get with the i7 processor, but looking at stores on line, they only offer the 5 processor.

Why would this be?

Just trying to do my homework.

I use a mac because it is totally reliable. Virtually no fear of viruses/bugs etc. like the regular ibm platforms.

I loved the 8 hour battery when new. However, now, I must have it plugged in constantly.

I work from home, all my work is via the internet. No games on my computer, but I do watch nextfilx, etc on occasion on line.

I am also looking to set up a 2nd monitor for my work and is there such a thing as ten-key pad that can be added (externally) that would work on my mac?

I welcome any and all comments and suggestions.

Thank you in advance for your time.