one thing about a shafiroff engine is it is what he says.It will perform as advertised.My friend John Graham in Calgary bought a 9.90 motor from him and it never had any problems.Standard freshen ups after 2 seasons(or less) depending on the activity.John doesnt just compete,he has been track champion several times and always places. That of course is more driving than engine,but you need reliability and consistency.
What I would do is;
price out the parts listed for the build and consider if you would use the same parts if you built the engine yourself.Then see how much profit the builder is making.
Do not consider the discount he might get because it does not apply to you.If you can replicate all the parts and have the engine machined a lot cheaper and you feel you have the experience to assemble the engine as well then you will have 2 choices
factor in when you will get the engine.and see if there is any warranty? Not selling Shaf engines,but he will sub out parts if you ask,usually to upgrade