17k should be a small part of your final portfolio, like just 2% say, or that ballpark. So if you mess up by buying RIGHT NOW and things drop 10% tomorrow, your mistake costs you 10% of 2%, or 0.2% of your total portfolio.
You will be buying many other lots over the coming years, so timing related ups/downs should mostly average out, and if you buy early you capture as much of the average growth as possible. If you fret and sit on money on average of a couple month to spread it out you lock in about 1% overall average growth (2/12*6-7%). So is locking in 1% less portfolio worth it to possibly avoid losing 0.2% this year?
Finally, the biggest risk is that something comes up, and you end up spending some of that money instead of investing is because it burned a hole in your pocket. Then you lock in a 2% hit.
If you are so freaked out about the market being frothy or some such, so much so that you don't trust the market with your latest contribution, then why do you trust it with your older contributions? Only if I was convinced things were really going in the dumper would I not invest, and I would use the EXACT same logic to pull out ALL my money and stick it in apocalyptic style investments like cash/gold/guns/ammo/TP/etc.