For what we use them for, there is no difference other than age, preference and aesthetics. But, yes, the main difference between the RB2 and the RB3, 4, and 5 is the stich length adjustmemt and reverse
The RB2s have been around for over 30 years and many of them ran in factories 8 to 10 hours a day for most of their life. I agree with
@sojourner - this one looks well taken care.
What type of servo motor is on it? You have to take that into account as well, along with table condition and any extra feet, binders or accessories that come with it.
Be careful. You sound like you are past kicking tires and close to the danger zone of owning another machine!!!! If you back out now you may dodge a hit to the bank account.
We have all been there......Be sure to upload some pics when its loaded in the truck!!!! Good luck!