When I first got my MIM strat, I replaced the stock pickups with Fender Custom Shop '69 Strat Pickups and I really dig them. After playing around with a HSS Strat and really noticing the difference with the humbucker, I was curious what my options were to try to convert to that.

So, I was wondering if it would be worth it, and any other info about them. I don't know much about pickup wiring, so I'm not sure if I could keep the two single coils and just replace the third. Also, what pickup to use for the humbucker, etc. Thanks for any information.
Most people like to use 500K pots for humbuckers rather than the single coil's 250K pots, but since the start only has one volume, I don't know what you could do there. I'm not sure of the difference, but I think you can use 250K for humbuckers if you'd like.
If you're getting a full size humbucker, you'd need a new pick guard as well. I'm pretty sure the body is already routed for a humbucker, so that wouldn't be a problem.
If you're getting a single-coil sized humbucker, just throw it in. I hear there might be a tone difference in comparison to full sized humbuckers, but don't quote me on that.
I'll share some knowledge if you do...

- You'll be fine keeping the other single coils in there, though RPGoof is right about the pots, I think they will behave more like switches when you're using the humbucker. I'm a huge fan of single coils so I don't know too much about the different flavors of humbuckers but I personally know a few people who swear by the SD hotrails pu, which is a single coil sized humbucker(if your guitar isn't routed properly). As for the wiring, trust yourself to do it, its really not that hard, just take your time and read a lot before you start doing it. Also, take a few digital photos of the insides of your guitar if you can so if you do run into any problems you can put everything back to stock.

- I'm considering a set of '69 RI's for a strat im building. If you could tell me a little about your experience with them I'd appreciate it. What artists do they remind you of?
While its true that 500k pots are optimal with full size humbuckers, you can still use the 250k thats in the guitar. If you really wanted, you could wire the 'bucker to its own volume and tone pots, but I don't know if the pickup selector would still function normally.

As for what pickup to get, you can always go with a stacked humbucker (single coil sized humbucker), but they dont quite sound the same as a full size. What style of music do you play?
The SC sized Hot Rail type option is not a bad one. I don't think they are as true to a real humbucker tho. When you said you played an HSS are saying you played a different guitar - Fender HSS - and like it. If it was the stock humbucker then fine, but I did not care for it.

I replaced mine with a Seymour Duncan Custom Custom Trembucker and love it. It has a nice Brown sound without being overly distorted. Also, I'm using a 250K pot and have no issues. They make a lot of humbuckers with different outputs so it depends on what you want to play. Oh, and keep your single coils for now and yes you will have to get a pickgaurd but they are like $20 bucks. Most all MIM Strats are HSS routed.