i should think you can buy a pc with windows 10 pro on it and if not then you could upgrade from whatever version it does have to the pro version easy enough.
would the flash drive you made install win10 pro if so then yes it can be used with the win10 pro key you purchase, pretty sure though when you buy win10 pro it will direct you to a download link to download the iso image anyway, that is if you didn't buy a retail version that comes on a flash drive anyway.
I am told i5 is on the way out and to get i7 but I need to take it ez on the wallet. Also, I would use the on board video card until i could save up.
no idea who told you that but they are wrong.
for what you do on a pc a new i5 skylake system would be more than enough power for you, in fact you could get away with building an i3 system in my opinion.
if you want to go i7 then that is fine, i say you can never have to much processing power and it's always better to have more than you need than not enough for that odd occasion when something you do on a pc could benefit from it. 
when you build your own pc the motherboard will come with a manual that you MUST read it will explain everything you need to know, all connections, etc. etc. also the case will come with a manual to help install things into that.
by reading both you will find the actual build is a very simple step by step process, needing only a simple phillips (crosshead) screwdriver usually.
install cpu into motherboard
install cpu heatsink and fan.
install 1 stick of ram.
place on motherboard box connect keyboard and monitor and power it up, if it powers up and posts then you know everything works, disconnect power supply, keyboard and monitor.
now install psu into case.
install motherboard into case.
install rest of your ram sticks into motherboard.
install dvd and hard drives into case.
now follow manuals and connect case connections to motherboard.
connect all needed power leads.
connect all sata leads.
close case and your finished.
now plug in psu to mains and boot it up and install your os.
if you need any help during the build of course we will help you.
if you need a parts list then give us a dollar amount you have to spend and we'll see what we can come up with.
also what sort of size pc would you like, personally after having massive full tower cases with water cooling and everything, i'm now into small quiet pc's. 
just built my daughter a very compact skylake i7 htpc to go under her tv. here's the parts list for that i posted on another site.
i'm in the middle of building and testing a new high end but lowish power itx htpc for my daughter, total cost about £1000.
some of the parts list here :-
cpu :- i7-6700
motherboard:- gigabyte z170n-wifi
case :- silverstone mlo5b
psu :- silverstone st45sf
memory :- 2x4gb hyperx black
storage :- 2x2TB samsung m9t
boot drive :- samsung 250gb sm951nvm
tv tuner :- tbs 6205
remote :- flirc + oneforall urc-7960
http://forums.pcpitstop.com/index.php?/topic/205553-im-jealous/
you could probably build yourself a similar i7 skylake system for about half of what she spent because you won't need half the parts. i'd not recommend using that case either unless you wanted a nice compact pc.
makes building it a pain because there's not a lot of room to place things, unlike in a nice midi size case that can fit a full size atx motherboard and psu in.
also if your building yourself a new pc, you could probably use most of the parts from your old pc to save you money.
i'd probably only buy a new psu, motherboard, cpu and ram to start with and use the rest from your old pc, maybe add a nice fast ssd to install the os onto and use your old hard drive/s for storage.
