The good thing about a AR is that you can build it piece at a time without selling a gun in order to finance buying a full AR at once. It took me about a year to build my AR(223) but I did it without selling another gun to finance it. I bought a stripped lower first, than the parts for the lower, than the bolt and bolt carrier, and finally a complete upper since I didn't have a barrel wrench and head spacing gauges. As many good diagrams and instructions that there is on the net its not hard to slap one together plus it's fun to tinker with a gun, by the time your finished you will know the AR pretty well. If you like your 45-70 keep it and slowly build you a 450 would be my advice, that way you can have both.
Dave