MP5 kit and laws

ok I'm a new-be here i have a few question

i want to build a MP5 short barrel and i do all the work my self ok my question has to with the laws

can i build a MP5 9mm short barrel legally my self ? (of course it will be semi not full auto ) i called a few people up who sell kits kept pushing me to use there gunsmith and would not answer my questions

to me this is part of this hobby sport it to make build repair reload your own stuff i tied looking for the laws on this but its misleading so ill ask you guys help on this

so if i get a kit and flat and a weld kit what else do i need to get or do ?

thanks