its not that bad to get a musket here, (uk) tho it does depend a bit on the police force where you live.
basically its just a shotgun licence. (smooth bore, under 51mm bore width and over 2 feet long.)
you will need a gun cabinet, umm the best way to start is to contact the firearms licensing officer at your local police station, theres a form/fee and then you have a home inspection to prove that you can safely store it.
im a re enactor, so our muskets are purchased with that in mind, so i dont know if there would be any complications in having one for non re enacting purposes, you might need to be a member of a gunclub so you have a place to shoot it.
i do think your 200 pound budget is probably on the low side, there all handmade nowdays, you can try derbyshirearms (wont let me link) they make nice weapons, and it will give you a idea.
oh another thing is youll have to get a blackpowder licence, again its dependent on secure storage, (another cabinet, explosive proof) but the fire arms officer might be able to help with that also, used to be the fire brigade, but im sure the cops issue them now too. depending on where you live youll be allowed a different amount.
while you can get modern propellants to use, blackpowder is the recommended, the guns are proofed for blackpowder only, and modern propellants because they have different combustable properties have been know to blow the gun up, i know someone who lost a eye when his musket exploded, it wouldnt have done if he'd stuck with BP.
A bit of a ramble but i hope it helps.