"Hi I am going to purchase a new marlin lever... this will be my bush gun for moose, elk, anything else I decide to hunt in the bush!!"
You didn't metion it, but all tests I've seen over 30 years show that higher velocity smaller calibers like the 30s are actually better bush guns if one is taking about getting through leaves and stuff. They tend to have higher stability. The real takeaway message though is nothing shoots through veggies worth a darn, so don't even try. Presumably you just mean bush gun as in short, handy, repeater though.
"I am going to go with open sights,"
So what are you figuring your max range and accuraccy requirement will be anyway, more is always better, if not necesarilly by much.
"I also am not a reloader. Should I go with the 45/70 or one of the marlin calibers. Ammo cost is a concern as I shoot all of my rifles frequently."
45-70 ammo is expensive and really low powered, it least what I have been getting. It really begs for handloading. The excitement, except amoung BP guys, for the 45-70 is all reloading related. You could shoot the hornady ammo in any Marlin though, so you would pick a little up there performance wise.
"Also what is the range and long distance accuracy like with the 18.5" barrel."
Accuracy on the Guide Gun always gets top marks. But realistically it isn't a long range proposiion, except maybe with a laser range finder.

For a gun I was going to shoot a fair bit, I wouldn't have a short barrel. For a self-defence only bear gun possibly, since the blast hurts him more than me.
Odly enough, given you don't reload, you might even look into the Legacy/Puma 92 in 454 Cassul. Neat little gun good price. Stronger action, longer or shorter barrel and overall length, small package. Good pricing in stainless, good accuracy.
Thanks for your help all !!!!! Scott