I'd recommend not fighting it. Or at the very least, not fighting it within its lair.The dragon likely has significant resources and you've said it has ample warning of your arrival. There are a lot of things it can do that won't be disrupted if you delay for 20 minutes before entering, or even 20 days. You should expect lots of permanent or near permanent defenses at least on the dragon's inner lair if not the whole complex it's lairing in. Mordenkainen's private sanctum has a 24 hour duration (or permanence). Possibly permanent prismatic walls erected just inside the sanctum, if you're hasty and walk into the shroud of the sanctum you hit the wall. Dimensional locks blanketing the area (19 days duration each for a great red wyrm). A minefield of various symbols to wade through. A widened antimagic field would perhaps be a good option for the dragon given the almost mandatory reliance on magic to defeat it. And if you do manage to overcome all of that and defeat it hope that it was confident enough to assume that it would never be defeated and so hasn't tucked away a clone of itself somewhere.
Whether or not the dragon would be able to do all these things depends a lot on the situation, of course. From what you've said, though, I'd be anticipating the worst. A great red wyrm would be tough enough for your group on an open battlefield. Add to that the fact that you're meeting it on its home turf. Add to that the fact that it knows you're coming. As you already know all of that (and your characters probably aren't ignorant of a lot of this reasoning either) I'd have to ask why you're doing this in the first place. Seems a lot like walking into a death trap.