Learned to model, what to do now?

So this is how it works in real life: 

• Rents are based on a market study of the submarket. You compare what exists to what you're building and what your strategy is (top of the market, undercut existing, be on par with existing price-wise but with a superior product, etc.) Typically you, or your analyst, does this initially, and if it looks good, you pay for a professional 3rd party study before you close. Rents always grow at 3%, like C.R.E Shervin says, although I'd counter that expenses always grow at 2.5% ;) 
• Expenses are based on historic expenses with similar product and construction types + some help from your broker buddies on deals they're selling or just sold + spot checking certain metrics. 
• Cap rate is whatever brokers tell you it is and is more or less whatever cap rates currently "are" in your market. You call the brokers, shoot the shit, and find out. Often, you get invited to golf, drinks, and/or dinner. You try to be optimistic on the cap rate, because your lender and equity partner are going to stress it anyhow. If you don't get an invite, there is always another broker who will be nicer to you. Make sure to spread the wealth around and play the field. 
• Debt/Equity terms are from a debt/equity broker or simply what they "usually" are from your preferred equity partners or lenders. You call them, shoot the shit, and find out. Bankers don't invite you to play golf, but lunch is a possibility. You typically want a harem of bankers too in order to better negotiate terms. 
• Hard cost estimates are from your GC. Expect them to tell you a lower number than it'll actually cost because they're GCs. Your GC will initially invite you to do some country shit like go quail hunting, but often you can settle on golf. 1-3 GCs will do initial estimates/spot pricing for you. Again, having a rotation is useful. 
• Soft cost estimates are from what you typically pay for those items + estimates from preferred vendors. Your architects and interior designers don't typically golf, but they love happy hours. Design-oriented gay men and straight women know the best happy hour spots. If it's the interior designers, you're paying though. They're not picking up the tab. 
• Tax comps are from similarly located projects of a similar construction type that delivered recently. 
• Impact fees can be a pain in the ass to find but most municipalities have them listed online now. Make sure to double check these - they vary wildly. 

Then, after all of that, for us at least, you look at the exit year Yield (or Return on Cost) and the JV Partner IRR. The yield needs to hit your company's target and the Equity IRR needs to hit what investors are looking for. (For us right now, a 6.75 yield is solid and a 17%-18% IRR will get our equity to bite off on it.) If either of them are close, you dig into the numbers a bit more and massage them until the deal works or you realize it doesn't.