Well I have no chance of being successful there then but thanks for telling me that. I have more confidence to build a computer now - that was back in 2012 I bought my newest computer.
Is it possible to buy a pre-built portable N64 from anywhere?
try ebay... some folks made their own portable consoles and then sell them, but this can be a really expensive and the quality depends on the person who made them...
you can also use an android device get a controller or an adapter and play it on emulators...
there is also some portable consoles which can play n64 games via emulation but i am not familiar with them
i totally recommend you to do some easy stuff like building a pc, or learn how to repair old stuff, like tv's (which usually is just replace some components) then with that experience you will be able to do something more advanced like build a portable console
for build a portable console there are lots of challenges, like the design, you will need to know how to make it "blend" its not so simple like find a box and put it inside... you will need to find ways to make it smaller, otherwise you would just grab a big box and put the console and the screen inside with some holes for the cartridge and controllers which will not make the thing portable at all...
what would be easier since you got a lot more of room, is build a n64 arcade machine, you will just work on the outer design of it, and put the console and the tv inside, make some extensions to plug stuff like the controllers but i am not sure if you would like this project...