go to this link, scroll down to Dodge. Scroll down to the year you are interested in. Lots of good cars to compare against.
also check these guys out: texastoybox.com
There are nice cars periodically on ebay but much of it is also junk that is offered by "dealers" as a "resto" but if you don't know fender codes it's easy to get ripped off.
There was no 70 "Rallye" option. It started in 72. 71 was the last year for the RT.
I owned a 73 Rallye Challenger, 340 / auto. It was a blast with the small block and I wouldn't hesitate to stick with one.
On the fender tag of any factory Rallye optioned car you should find the code "A57".
There were no "portholes" on any version of the car, in fact I've never seen portholes on any Mopar except a van. The 70 Challenger SE had had a smaller rear window than the standard Challenger.
A 73 Rallye with numbers matching components isn't going to be a high buck (depending on your definition of high buck) collector item but it is a very nice car. If you get a matching numbers car don't alter the components (i.e. swap out the engne or trans. - if you do then keep them in storage).
If it's non matching or just a 225 or 318 flat hood car then indulge yourself.
The resto topic comes up pretty often in here. My 2 cents to you is: if you're not in it for the money then buy a shell, strip it down and build it right, the way you want it, from the ground up. Make sure you have thought thru what it is you want to do with the car, it always costs more to do things 2x.
If you buy someone else's "driver quality" car you will probably end up redoing much of the work anyway and you'll have spent more up front just to begin again. Ask me how I know this.
The sheet metal for these cars is readily available so body panels aren't an issue.
For engines that are ready to go check out: cmengines.com