I forgot to address your question in regards to "left hand".
First off, no question is stupid and never be afraid to ask - the only stupid question is the one left "unaasked".
If you are ordering a barrel - say a Green Mountain or another brand - it will usually come with the muzzle "crowned" and the breech end threaded. The threaded end will have to be fitted with either a "patent breech" or a standard breechplug. If you are going to make a left handed rifle (lock on the left), it may and probably will be a problem coming up with a left handed patent breech. If you use a drum and nipple (or if you are making a flintlock and installing a "vent liner" - you just drill and tap the barrel on the left side instead of the right. Basically you are building a reverse of a right handed rifle so you locate the barrel channel, lock, ramrod hole, cheek piece (if you are going to have one), capbox or patchbox (if you are going to have one) on the reverse of where they are on a right handed rifle. Most locks are available in either right or left hand and you use the same trigger (single or set) on either a left or right hand rifle.
If you are going to build your rifle, I'd suggest getting one of the excellent books that are out there on constructing rifles. While you may be building a different "style" than what is illustrated in the book, the construction basics are the same. Not knowing your level of skill, you may want to look at getting a "precarved" stock with the barrel channel, ramrod hole and lock mortise already cut for the barrel size and make of lock you are going to use. Since they are also "contoured", it can save you hours of work compared to using a rough stock blank and starting from scratch.
Parts are available from many suppliers but I would recommend that you get a copy of Track of the Wolf's catalog. Even if you don't order from them, their catalog shows most of the current parts available from the lockmakers, etc. and they are illustrated "full size". This can be a great help when trying to layout a rifle and deciding what makes/parts you want to use as they can be easily measured from the illustrations.