Dave, the decission for you is obvious. It's a no brainer here since you know how to build a Serpent and the best part is that you have lots of Serpent spares which I can bet that even if they are specifically for the Impact, some will fit the 705 as some of the parts are common
The 705 actually has some parts that are borrowed from the Impact like the pulleys and all.
From the things you say, I get that you know how to make the Impacts run on the track... So there's actually no worries to get the 705 running. I'm pretty sure you know how to set the car and also how the Centax clutch works. Yes, Punisher is right that there is a lot of bad rep going around about Serpent cars being not good. It's just sour grapes that Serpent cars will go round and round the track without breaking stuff. They just can't help seeing that the 705 cars do not need hop ups and probably do not like get beaten by them.
You do not need anything else to get it running. I believe if you have built and maintain an Impact, then you would have almost all the necessary tools to build the 705.
Yes, like Punisher mentioned, get some spurs (different from Impact as they are now lightened and the same is used on the 835 and 950) and pinions (they are the same as the ones that are used all along).
For a short track and technical track, I would suggest you change the brake pulley from 23T to 22T and diff pulley from 46T to 47T just like the Impact
You will also need a 17T rear side pulley to replace the 18T stock rear side pulley. The 47T diff pulley is a part from Impact and you may already have this part. The other pulleys I think are new parts. Feel free to shoot questions and we willl be more willing to help you. Good luck