you should avoid buying a custom bot, you might encounter a scammer or waste your precious time on something which already exists.
followliker seems to be the best twitter bot at the moment, they have a BST on this forum (just bought it)
it's 47$ for 1 account, you can also pay more for an unlimited account.
it can follow people from a certain keyword or certain account and it can focus on accounts that tweet regularly and have profile pics (which means they are active)
it can tweet WITH images (not all bots can do that), unfollow... basically it can do everything you need.
but you can also avoid paying for a bot by using tweepi(to follow and unfollow users in bulk) and twuffer to schedule your tweets.
it really depends on your goal, the amount you want to invest etc...