you can keep a bull with the cows but you will have calves year round. not necessarily a problem, lots of people do that.
if you want calves at a specific time of the year, you need to be able to separate the bull from the cows until 9 mths before you want the calves born.
if you do decide to leave the bull with the cows and have calves all year round, you might still want to have an extra pasture to put the young heifers in, so the bull doesnt breed them when they are too young
also, best to buy some older cows - tougher than heifers - you can buy what is called a 3 in 1 cow, that means she has a calf at foot and in the belly also, so you are getting 3 animals for the price of 1. Buy them, forget about the bull for a while, wean the calves off at the appropriate time, wait till the cows calve again, by this time you can load the weaners and take them to market, use the money to buy or lease a bull, turn him in with the cows at the appropriate time for when you want your calves born.
Good luck with it all! :mrgreen: