Trouble finding a duplex... Build one instead?

[Alyssa Healey]

I am a pretty new investor, I have two properties in Denver currently. My market is very expensive, and I have been struggling to find a duplex that makes sense... I found an off market property that I could buy and it is zoned for a duplex. The property will cost around 300k, I could buy it as a single family rental, and probably break even. 

If I were to have a brand new duplex on the lot, it would rent for around $5500/month. I have no experience in new construction, and am not sure what the costs for this would be. My question is would it make sense to tear this property down, and build a brand new duplex? Or how do I go about finding out what it would cost to build one so that I can properly analyze this? I know that there are a lot of different costs to consider with a new build, and I am not sure how to find out what all of these costs are. 

Another option would be to keep the current structure and build onto it, it is just very ugly, so I am not sure if it is possible to add on and make it look nice...

Any help would be greatly appreciated.