First let's talk about the troops. Hand flamers sounds like a great idea, but are only ok at best vs. toughness 3. I find it is worth giving up that extra CC attack to get real flamers and real melta guns over hand flamers and infernus pistols. If you decide to go with priests, power weapons work out better than power fists. The priests are giving you furious charge, upping the strength and initiative of your attacks, this bonus is pretty much lost on the powerfists, since it doesn't help with the initiative and upping Str to 9 is not much difference than 8 in CC.
For dedicated transports I'm, a huge fan of either Rhinos or Heavy Flamer Razorbacks. The transports are going to get shot up quickly so spending more points for the big guns on top is a waste. Transports exist to get your marines 18" on the first turn, so don't over spend here.
Flamestorm Baal is good. I disagree with Ben on adding sponsons though. The template is only about 8", so the Baal is going to be moving 12 or slightly less each turn to get into position. This means that you are only going to be firing one weapon each turn. The points are better spent elsewhere....heck even Extra armor on the Baal would be preferable to the sponsons (Sorry Ben I think you missed the boat on this one completely). Baal with the twin assault cannon should definitely have the sponsons though. Tons of medium strength shots for a decent price. This works great against light transports and meq infantry as well.
Mephiston is awesome, just remember to keep him out of site or in terrain. No invulnerable makes him squishy against melta and lascannon shots.
Not sure about the two Dreadnoughts in the heavy spot. The assault cannons are a good weapon in every game, but I find they are never a great weapon. I'm a huge fan of the Twinlinked Autocannons x2 on two dreads and keeping them in the back lines as transport killers.
I have no opinion on the bikes, haven't used them in a long while and only with Dark Angels and Vanilla marines lists, so please share how those end up working for you. Keeping a priest near them might be interesting, Toughness 5, feel no pain!