I cannot answer your question about square feet amount to range on a SF cost of apartments but that really matter I you have a deal or not.  

I don't know wha market you're in, but 42k would be enough to buy a home in the 200k range.

.$450K house in Dallas is not considered as investment category. i recommend to sell and buy house in 150K - 175K range with rent in 1400 - 1600 range... following are disadvantage of renting 450K house...You are getting >0.8% rentMaintenance like repair/carpet/paint will cost lot more in higher value houses.

Is your property middle of the road with respect to other rental comps, or is it at the top of the price range.  

If yes, my price range is around $200-400K based on availability of down payment.When should I consider a 1031 exchange of the existing Colorado townhouse (3/2.5)?  

The latest quotes I'm seeing come at 5%-6% range.

The house I am interested in is listed to sell at $182,000 while other houses nearby (comps) are at around $140-$150k range.

@Sam Hiatt depends on what price range you are looking at, but my threshold is anything with that cap rate and >=$300 cash flow per door.

I think that you should be looking for anything that is in your price range and the numbers work.  

Our target price range is probably below what you are buying (30-60k), if you come across any lower price point deals feel free to reach out.