Hi Steve:
Welcome to the wonderful world of HO.
My take on your question. There is nothing to fear regarding the MTH SD70ACe locomotive. I know two "HO Purists" that purchased the MTH product, and are very complimentary of them. The metal handrails are a big plus. One even went so far as to put operating cooling fans into his (he is a retired EMD design engineer / manager). I believe both of them did change out the MTH decoder (dual mode, DCC or DCS) with SoundTrax Tsunami 2 decoders, which were state of the art sound decoders when the MTH ACe was first released.
Current state of the art for DCC decoders, in my opinion, are either the LokSound 5, or the TCS WOW series. I have standardized on LokSound for my diesels, and TCS WOW for my steam. I haven't changed everything yet, but as other decoders fail, or I upgrade, that is what I am going with.
I wouldn't touch the DCS system for HO, and it was one of three big reasons that I bailed on my three rail scale modeling six plus years ago, and decided to stick with HO, which I have been playing / modeling in since age three.
Nothing wrong with the Athearn product, except the first run ACe locomotives did suffer with poor handrails. I think the later releases have improvements, but you should check with your local hobby shop, or Athearn direct. I model 1952, so I have no direct experience with this locomotive, except in my day job (one reason I do NOT model modern era).
To answer your second question, yes, you can run either the Athearn or MTH with a conventional throttle. You had a recommendation to go with NCE Powercab for a control system, and I would second that endorsement. I'm a 26 year user of NCE technology. The owner, Jim Scorce, is an O scale modeler, and posts occasionally in the two rail forum. I have old NCE (and Wangrow, which were the same) throttles, and a power supply that are still my original purchases. I've since purchased a lot more later production NCE, particularly NCE Master Cabs with radio.
Regards,
Jerry