P.B., the new Marlin trapper is a good option right now IMHO. Just went down the road you are on and debated b/w new vs used-customized 336....I bought the Davidson's limited edition trapper off Gunbroker through "RoybalArms". Jesse builds AKs but also sells other firearms great guy, honest, and gives military discounts. His current add on Gunbroker lists it for $699. With this price, you get a black/grey laminate stock (really nice looking), 100% stainless metal, Williams' brand fiberoptic sights, only 500 made/collectibility, no need to hassle with/pay a gunsmith to customize an 18" 336.
I had a bit of sticker shock initially with this Trapper model, as I own two standard blue/walnut 336 30/30s and a 45/70 guide gun, which cost much less. However the extra cost is acceptable to me considering the aforementioned factors. As far as accuracy, you'll get what you are looking for within 100 with open sights....the 16.25" barrel may cost the bullet some feet per second over the longer barrel models, affecting our longer range ballistics...however the cut down 336 is not meant for long range hunting/shooting. I am a fan of the Hornady Leverevolution gummy tips as recommended by Furncliff, however the cost of theses rounds ($22-25/20) is not going to necessarily gain tighter groups at 100 yards. Your new trapper/or customized 336 may group the cheap Wally world Remington Core-Lokt's ($13/20). If you reload, yours may like Sierra 150/170 grain bullets over IMR 3031 like my 336s...You'll just need to experiment one way or another. As far as the 45/70....sure you can pick a blue/walnut one up for $500 at Bud's right now with similar features to the Marlin 336 trapper, but the thing is a cannon and ammunition cost is HIGH!!! Good luck with your purchase decision.