Your Zenith kit will easily fall under the 51% rule, meaning the assembly of the kit by any amateur or group of amateurs is a non-issue and you won't have a problem getting a DAR to issue airworthiness. As mentioned the repairman's cert is a different bird and only needed for the annual condition inspection on that particular air frame/plane. If you're not heavily involved then you probably shouldn't march down to the FSDO to try and get this. If your buddy feels he'll know enough then he can do that but really the only upside is he can then do the condition inspection on it... otherwise it has to be overseen by an A&P IA. So farm it all out and get your plane built and airworthy, just don't try and get the repairmans cert.
Now you'll hear lots of opinions, mine included, on how to define amateur vs professional. Bottom line is the FAA doesn't do a clear cut job of defining this, other than saying for education and recreation. Be it paying someone or not, they're most likely learning something or maybe they just enjoy doing the work even if they're being paid. So pay people to help you, the FAA has never made a clear ruling on this or how to define a 'professional builder'. If they had a heartache with it then "builders assist" programs wouldn't be a thing. Under the same premise you wouldn't be able to have school programs which build planes, teachers typically are compensated. So have your friend or other people help you, buy them beer, pizza or give them cold hard cash. Just make sure that someone involved with the building is there to answer question for the DAR when they come inspect the plane.
On kit plane specifics you have to go off the knowledge of previous builders. Everyone will tell you adding/changing items from the plane will involve more time, complexity and typically money. If there's a mod out there for an RV-10 chances are I either have it or at least considered it for my build. You should be able to find reviews of both the Rotax and Vikings engines. More HP isn't always better since it affects weight, heat and a number of other factors along with additional horse power. Remember it's your project so build or get it build how you want.
Lastly I think by 'trading' you mean buying and selling or maybe you do mean straight up trading someone airplanes. Once the plane is airworthy and have completed all test flights it is treated as any other airplane. You sell or trade it that new owner still has the airworthiness it had before. I don't put much stock in the liability argument, not saying it's impossible to get sued because frankly you can sue anyone for anything.... however the chances of actually being help liable for some type of negligence is slim, especially if it had a good number of safe hours prior to sale. You could go as far as have them sign a release of liability to make yourself feel better, in reality you can't sign away your right against someone for negligence. If I was ever to sell my RV-10 I would draft up a release of liability and be very specific about the accepted risks.
