I believe, today, it is possible to submit Design Challenge with total costs only 1350 design coins.
Note: Small Fire Station Lvl.1 has capacity 50 (health and police have more.) and some services are provided free at the beginning.
GreenValley home lvl.4 has demand 5 and population 356, so you just need to fit 10 of these homes , and you get total population 10x 356=3 560. That population is sufficient to Submit the design challenge.
Costs:
3x Deluxe Wind Power Plant lvl.1 = 3 x 200 = 600 coins
1x Water Pumping Station lvl.1 = 750 coins
No service upgrades needed and total costs are 600+750=1350 design coins.
I guess, I do not understand what is your problem.