Same goes for my dutch Div. II league. I'd say build more bleachers early on instead of the lower tier because they sell so easily, and stop building them at 13k. I agree that you'll need much more lower tier later on to get the the ideal arena, but you don't need them early on because they don't bring in that much money for the building costs if you build that many.
I always build up gradually, keeping in mind the return on investment. If I have $70.000 and I can chose to add 700 bleachers that sell for 15 or 100 lower tier that sell for 30, I know what I'm building.
And then when you've got 13k bleachers and, say, 2500 LT seats, and you promote to I, hooray! I'm sure you'll have $2.8M lying around to spend on adding those last 4000 LT seats, and wouldn't have anything better at all to use it on. ;)
Return on investment is one way of looking at it, sure, but another way of looking at it is that at some point you're going to invest X dollars total in building your stadium, and are you going to have better results investing while money is easily made or when you're scraping for every dollar you can?