If you continue to unlock lands, place guilds on 'em and then save up for your gigantic conversion, it'd be easier, IMO. I've seen it done many times. As a t3 hansel, you should be sitting on a gold mine, anyway.
Convert but start at land cost 40 Bil or so. Then do conversion per land at highland to t4. Once hlbc,,convert lowland. Or do all now and build up slow . FYI , I would and have on alt. done em one by one. Slower but works out in the end. Belle guide is cool too.
When your primary source of income is haunting, the plunder drop is about 2-3mil (for my case anyway). So converting gradually isn't that taxing on your plunder. But if you want to avoid the drop, I would save just enough to convert 2/3 of your guilds to SOS in one shot. That way you don't have to save as much if you were to convert all of the guilds to SOS. But saying that, I would suggest you do it gradually
Oops, didn't even answer your question. I would open as many lands as I possibly could. But don't open up ALL the lands though as that would take too long. Perhaps you could stop at the 40b or 70b land. The more lands you can possibly get means the more capacity for higher plunder. Once you establish a good income (which is quite easy with hansels), it'll make it a lot easier to then explore the rest of the lands.