The SF2 unlock is the autosteer unlock. It's like the $3000 you paid for your SF1 autosteer but it is for SF2. You'll need to upgrade your current system to SF2 in addition to my list before adding RTK.

Base- 
         Used Gen. II reciever - $1800-2200
         RTK Activation          - $3000
         Radio                      - $1400
         Big Antenna & wiring - $500

Rovers-
      1 New ITC SF2 Reciever - $4500
      2 RTK Activations         - $6000
      2 Rados                      - $2800
      1 New display              - $2000 or 5000
      1 SF2 Autotrac unlock    - $7000

Upgrade current unit-
      1 SF2 Autotrac upgrade  - $4000
      1 SF2 Reciever upgrade  - $2000

                                          $35000 - 38400

Big leap to take in one year. I think I'd get everything setup for SF2 and then finish it in a year or two.