First of all, get prepared with paints.
Army Painter Black spray prime is the one I use for my top quality. Get some white too, that will make your life easier if you just want to paint quick. Get yourself a copy of the White Dwarf with Space Hulk in it. They cover a nice Blood Angels paint scheme. Or pick up my How to Paint BLood Angels video set off my estore. That set uses white spray prime and blood red as its main paint.
What is next? Some things that will always be Blood Angels staples are Speeders and Attack bikes. They are a good choice to buy, and I doubt there will be much in the way of upgrade parts for them.
A huge buy will be Scout Bikers. I just bought 5 myself. It is a brand new model and is a new addition in April. More importantly, if you are planning on using those fancy new landers, you will need Scout Bikers! They are a cheap way to get a homing beacon on the table.
Of course Tactical and Assault squads will be needed. But don't get too crazy assembling them. The new release will surely include Chest Pieces, shoulder pads, weapon and head options. If you feel like you must, assemble and paint half of them, but leave the other half free for upgrades.
Current Blood Angels stuff? Forget it. Don't buy any thing, including Dante and Mephiston until you see what may be in store for us. I doubt they will update Dante, but I have a feeling we will be seeing a new Mephiston model.
The Baal Pred is only being sold as a bits box now. I suspect there will be a whole new Predator tank with all the great options, sorta like the Land Raider Crusader. So don't buy any tanks. If you come across a Corbulo... maybe you should grab him. The new dex omitted him, but you can still build a priest like him using upgrades. I bet there will be upgrades in the new box sets, but he is a good model and worth having.
You can buy Ironclad dreads. They have all the furioso options ane are great models. I decorated mine with some of the Blood Angels icons from a drop pod. Go on Bartertown, you can probably get hundreds of em for nothing. Do it now before too many guys catch on!
Drop Pods are a great buy. They come with a BA decoration piece so I doubt they will use precious sprue space to include another.
That should get you started!
If you are the type of guy who needs defined list:
5 Scout Bikers
2 Tactical Squad
1 Vanguard Vet
4 Aassault Squad
3 Attack Bikes
3 Drop Pods
You can paint the pods, bikes and speeders without fear of missing out on the chance to use upgrade sprue bits. The others you will have for when you are ready!
Go shopping at the warstore or miniwargaming. Tell them Jawaballs sent you!