Exciting time, and welcome to our world! Hopefully your ASA instructor will have mentioned that the course work is only the beginning. Sailing is a physical skill that must be practiced for proficiency. I agree with your plan to put off buying any boat until you've been on several. It's like asking a 16-year old to pick out a car to keep for 10 years right after they finish drivers ed.
A 20-24 foot day sailor is great for learning (I still have one). There are several advantages: (1) Trailerable - you can explore different areas and build up to challenges; (2) cheap to store when not sailing - you don't have to pay dockage or haul fees, etc.; and (3) cheap to make mistakes on - like a 16-year old, you will damage the first boat you play on without an instructor. On a day sailor, even severe damage (torn sails, hull damage, etc) is a few hundred bucks, not thousands.
As far as chartering, there are many threads on this site that will advise you that you may need more than an ASA104 certificate and a credit card before a charter company lets you have a quarter million dollar boat in the Atlantic Ocean. You should start logging your sailing time, including on other people's boats, and build a sailing resume. It doesn't have to be very formal, but it helps when filling out their form to say that you've got 100 hours as crew on a keel boat, etc.
As a last piece of advice, I suggest you get involved with a local sailing club to participate in races as a crew member. You might not consider yourself a competitive racer (I'm not), but sail racing teaches boat handling, sail trim, and crew management. Most crew members in 'beer can racing' don't have half the formal sail education than you now have, so you're really qualified to participate. Most clubs sponsor weekday beer can buoy races that last about an hour and are designed so you start after work. See if someone is looking for crew, carry your inflatable PFD and a six of beer, and start gaining experience.
