Seems to me, all my life it has been said that it is better to buy old cows and sell the heifer calves. But I notice the good herds have been built on selecting your own replacement heifers.
Soapweed brought up a good point a couple of years ago...he sold cows and kept heifers as he could use Capital Gains against the cows and the heifers would be ordinary income. There is a tax advantage to doing that; but I don't think that works buying cows one year and selling them the next. I think you have to keep those cows for 2 years in order to do that.
If you are careful when buying old cows you should be able to get more than one year out of them. I know the old cows we have sold, those that bought them got 3 or 4 years out of them.
So, it is really up to you and if you want to take a chance on having some problem cows or wait and calve heifers. There will be some problems with those old cows, most likely. Bags, disposition, etc.
Good luck!!