A gentleman named Frank De Haas wrote some books about the old single shot rifles. Then he wrote one about building them. At Brownell's gunsmith supply site is (I think) a book "The Home Machinist...something"
You will find your own way. My first rifles were blackpowder muzzle loaders. Fun to build and shoot. Building the lock from a Siler kit was an interesting project. The "Track of the Wolf" catalogue is a USA source. An old but functional shotgun hammer lock can be used.
Its great experience.Fitting wood to steel,shaping,finishing,etc.
Modifying mil-surp rifles to sporting rifles has fallen out of favor,but it can still be an education.It does not make economic sense,but I am glad I built the rifles.
One of them was a 1927 Polish 98 pattern rifle made in Warsawa. No,I did not butcher an original.An old man offered me the receiver only for $25.
How easy is it to find a cheap SKS? You might dump the gas system and make that a straight pull. (Or a pump!) But shoot it some,first.Kind of a fun rifle.I had the carbine.It was $79 new in a box,and ammo was $69 a thousand..a while back.
I suggest its less about "the perfect rifle" and more about getting your hands dirty. An education.Skills. Your third or fourth build.They get better.The gun is not the product.You are the product.Skills,workmanship,experience,confidence.
A man name John Bivens wrote a good book on building IIRC,The Pennsylvania Long Rifle. A flintlock is interesting. A bit of lead,some black powder,flint,and you can shoot. Quite accurately.A good thing about a muzzle loader is,that you can get it done. Drill press work if you build a lock kit.A plumber's torch will let you do the mystery of heat treating the frizzen for spark and the sear parts to last.The breech needs threading for a breech plug,but often that's done already when you buy a barrel.Many nice rifles have been built in a back woods cabin with hand tools.One of the Foxfire books has Herschel House building a rifle that way.Great books. Teach you how to build a still and make moonshine,too.
Or find an old rifle,maybe a .22,that needs a little love.Make it fit you.Put on good sights.Restore finish.Shoot!
And as you no doubt have heard a thousand times,"Have fun,be safe!"
I dream a shooting tool,for a purpose.Then I build it.