Soooo, I'm buying a ke70 soon and I plan on putting a 3sgte in it. Is this a difficult swap and how much will it cost me in parts that aren't the engine itself?

Yes its difficult but awesome and probably worth it once done. IMO.

Need to make mounts, manifolds in and out, sump mods possibly, depends a bit on which 3s you use, as the stock 3sgte motor is front wheel drive and the manifolds are less suitable, whereas if you decomp a beams its already rwd and higher tech and maybe a better starting point cause you can use the stock inlet manfold. Have you bought your motor yet?

No, but I see them all over for less than $1,200. The beams is quite a bit more pricy