Another here who favors a boxed set + advanced books model:
- One core boxed set, which contains a complete, playable version of D&D 5E. Fighter/cleric/rogue/wizard, human/dwarf/elf/halfling. All the most common and general spells, equipment, etc. Pre-gen characters alongside fully functional character creation rules. The rules should be suitable for pickup games, but not intentionally incomplete. MSRP should be $20 or less.
- Player's Handbook: More classes, more races, more spells, plus tons of rule and non-rule options for personalizing your character. Focus on making characters memorable and interesting, more than giving them an overwhelming array of optimization options. MSRP should be $25 or less, sized to match that price.
- Dungeon Master's Guide: A toolkit for DMs, with a focus on how to produce campaigns, and how to produce your own content (spells, magic items, monsters, etc.). Include optional rules, including popular options from previous editions, but stress that they're optional. Needs to be as inspirational as possible. MSRP should be $25 or less, sized to match that price.
- Monster Manual: As much as I'd love a tome the size of the Monstrous Manual (the book that got me interested in D&D), that's probably not practical nowadays. However, a "greatest hits" approach is critical. I also think stats are less important than making sure the monsters are interesting both inside and outside of battle, so I would err on the side of more fluff - like the Monstrous Manual. MSRP should be $25 or less, sized to match that price.
Oh, and I favor 6x9, like the Essentials books, over the traditional 8.5x11.
I should note, BTW, that the playtest packets present a perfectly playable game, yet - as someone noted - they're a little over 300 pages total. We don't need huge books up front, those can arrive later, if they must (I think we can keep them smaller).
One more thing. When you buy the books new, it should also grant you access to online/app versions of the material. Free access would be best, but a free trial would be a nice runner-up.