Warhammer 40K Tabletop VII: Common Sense is not RAW.

[Gauntlet]

The Assault Squad comes with jump packs as standard, so that's where I'll be putting my Sanguinary Priest. The Priest gives Furious Charge and Feel No Pain to units within 6 inches- but Death Company already have both of those abilities, so he's just a glorified Assault Sergeant in that unit. As such the Death Company get Seth riding along in their razorback instead. The Twin-linked Heavy Flamer doesn't cost points- it's a free swap between that and the default TL Heavy Bolter (however, the flamer you have to find a model for somewhere ).
Jump Packs on Death Company are expensive. 35 Points a model. With a Razorback, I'm paying 35 points more than I would for an Assault Squad of the same size with the same transport, and in exchange they actually have the stats and potential to get something done on their own. I usually find an unsupported 5-man assault squad can't be relied on to kill anything bigger than a grot. Add furious charge and bump them up to 2 attacks each and I can generally be sure that they'll do enough damage on their own without just being 5 extra wounds for Seth.

Edit: I guess I could spend another 15 points and swap out the Death Company unit for Honor Guard, which have the same stats but don't have Black Rage, and would give Seth Feel No Pain and Furious Charge as well (not that Furious Charge is that useful when you're already S8, I5 ) On the other hand I don't really have many points spare. Probably best to stick with the Death Company and give my Assault Squad some anti-tank capability.