Night and day compared to the 670 S-chassis sleds- whether that's good or bad is entirely up to you. I'd say take some time and try to ride some other sleds first- from other brands too.
The change in power characteristics, and not just total power, will take getting used to as well. Well tweaked 670s are stump pullers and really disguise their lower total output vs. newer sleds pretty well.
Set yourself a total budget- purchase prices, plus repairs, plus intended mods, plus a reserve for unexpected issues and work from there. Lots of used sleds out there these days, but being patient and finding the gem is part of the adventure. With any 4-7 year old sled, your fighting age + wear/tear as much as just miles & price.
At 16, I'd say keep the 670- world's better than any sled I ever had until I was nearly 30!
