The Angels of Death book is nice - I no longer play SMs but I think that book is the best "supplement" book by far - it's mostly lacking fluff and provides a lot of "minor" SM factions get some love with Warlord Traits and Relics (basically all of the chapters with Chapter Tactics in the SM book). So if you can get it
cheaply then do it.
The other bonus is that the book contains the 40k rules for some units from the Betrayal at Calth box (namely the cataphratti termies and the contemptor dread)
I nominate the Betrayal at Calth
because it is a huge deal if you are planning on using the entirety of the box and are planning on playing 30k/40k SMs
1 Terminator Captain (in cool looking Termie armor) $25
1 Chaplain / Dark Apostle $25
30 Tactical Marines $50 x 3
5 Terminators (again in cool armor) $60 (I Personally would build these are pure Lightning Claw Terminators, despite that they are not really competitive)
1 cool Contemptor Dreadnought - fits into the Iron Hands theme too $65
Total price = $325 for sticker price if you were buying everything individually.
This gives you a good base of Tactical Marines (if you want more stuff you buy Devastators, Sternguard, and Vanguard boxes rather than just generic Tactical Marines). If you need more Tactical Marines, I would suggest either getting the $85 "starter
" set of a tactical squad, a venerable dread, and Termie Captain or looking to buy them second hand, preferably locally before ebay.
It gives you two semi-unique HQ models for you to use with a Termie captain/chapter master, and a chaplain.
It gives you a sem-unique terminator squad, and a semi-unique dreadnought to use.
Things to think about;
Dreads: I find it better to just buy the "Venerable Dreadnought
" instead of the standard one - it looks cooler, and it generally has the same options available in the box.
Rhinos: NEVER BUY A RHINO BOX, always buy a Predator, Razorback or something else (besides Vindicators unless your way better at magnetizing stuffs than I am) and then magnetize the parts that make it a Rhino and the Other box.
The other big box that has been released recently
is decent, not really a decent discount in comparison.
Hope that helps.