The following is just an opinion, so take it for what it's worth.
Lock placement is important to assure alignment of the hammer with the nipple, so yes, you should inlet the lock in a similar manner if you are maiting it up with a similar barrel.
As you didn't say you had a barrel, if you don't, it opens up the question of the nipple location. It could be on a breech plug or on a side drum. Both are PC.
When you are looking for a lock, remember, for about $75-$85 you can get a good brand new American made precussion lock.
Single Set triggers are hard to find unless you buy directly from a parts supplier like TOTW. Almost no guns come from the factory with these.
If you want a set trigger, I suggest you look for the more common Double lever, Double trigger style. This is the type that TC and most others use. This style will fire the gun using the front trigger, whether you cock the rear trigger or not.
If you get a Single lever, Double trigger style, the rear trigger must be cocked before it will fire the gun.
If you are looking at a trigger in your hand, the Double lever style will have a vertical blade on both the front trigger and the rear "trigger".
A single lever trigger will only have the vertical blade on the rear trigger.
As for style, if you like the Seneca, that's fine, but if your going to all of the work to make a new gun, you might want to consider some of the other styles of stocks which are available.
Bear in mind, you can buy a stock blank which is cut for the barrel and ramrod only. They only need to know what kind and grade of wood you want, the width of the barrel and the diameter of the ramrod hole.
This gives you the option of installing the barrel, lock, trigger and furniture of your choice.