Bitstamp is often referred to here as a trustworthy exchange and they are one of the longest running exchanges around today. What makes them so reputable is that they do not offer leverage, uptime is very consistant and support is top class. Most exchanges support API keys which is how you'd set up DCA buys. You may want to post in technical support to learn how to do this.
For Canadians, I really like
Bullbitcoin they offer DCA and it's a non-custodial exchange for Bitcoin only. Highly recommended but you must be Canadian.
There are indeed plenty of reputable exchanges to choose from and regardless of which exchange you decide to use consider these Pro tips, unless you're a experienced trader, try to avoid the urge to trade and do not keep any more value on the exchange than you're prepared to lose. Bitcoin or fiat.
Happy DCA'ing and congratulations on your journey into monetary freedom.