Anyways, what Dragon Quest game should I play to get hyped for his inclusion?
I'd say Dragon Quest 5 on the Nintendo DS is the best starting point for the series. Almost everyone I know says they tried getting into DQ to understand its appeal but could never quite get it until that specific game. So, Dragon Quest 5 on the Nintendo DS is my highest recommendation.
That said, to more directly answer your question since DQ5 doesn't have Erdrick in it, then you might as well play Dragon Quest 3. It's bar none the most influential JRPG of all time, and I played it earlier last year and I had a great time with it. I recommend the iPhone or Android version because the games are meant to be chipped away at a little bit at a time, over a long period of, say, a few weeks to a month. It's way easier to do this on a portable version than on a dedicated console, and the games are so simple that using only touch controls is a total non-issue, you're just walking around and navigating menus after all.
The number one tip I can give newbies to Dragon Quest is to not go in thinking the battles are going to be the thing to look forward to. They're not bad at all, it's just their purpose is to provide tension to your journey more than anything else, which some people misconstrue as "boring". The world and its characters, and your adventure exploring them all, is the real draw of Dragon Quest. THAT'S what hooks you in. That said, both Dragon Quest 3 and 5 have interesting hooks to their battle system, which is fully customizable party members and the ability to recruit monsters, respectively, which add a good amount of depth.
And as for the story, the overarching one is always pretty simple, but every game has a bunch of subplots that are interesting in and of themselves and leave a great impression, so again the DQ games are more interesting than they initially seem. Whichever game you start with, stick with it and you'll hopefully understand why they're so appealing.
(I didn't put Dragon Quest 11 because I haven't played it. That said, I've heard really great things, and it IS aimed at new players, but it's not portable, which is something to consider. A Switch version is planned to come out next year, so you can decide whether or not to wait for that one if you decide to start with DQ11)