1. If you intend on playing a
legion which featured at either Isstvan 3 or Isstvan 5 then you'll need both legion astartes army list and the Isstvan campain legion red book. You will be able to get away with having only the legion astartes red book but then you'll miss out on the fun of the Isstvan Campaign
2. The red books are your bare minimum and contain an army list and unit selection for the army list it represents. the black books are more expensive ~£70 but contain fluff about each force, their equipment, how they fight and what they tended to do in war. etc etc. (very useful stuff)
3. Cults/militia list; use your existing models, use the upgrade system to make them
wysiwyg and then fun ensues? Betrayal@calth is popular these days (£95) and will give you enough bits that you have to find someone else who also bought one to swap weapons with so you can actually Field a support unit or a heavy support unit.
Personal favourite is The Ordo Reductor with thallax as compulsory troops (£30 per cohort, £80 for 3 cohort bundle) recycle any old techmarines into techpreists/myrmidons and use the titan techpreist as Calleb Decima (£13) and 'requisition' some artillery tanks for some heavy support; maybe a landraider?
A good idea would be to get your hands on the relevant black book once you have picked an army and then go from there with the fluff.
btw: this idea of doing 30k on the cheap is a lie; we all know that it starts as that and then you start wanting to have a go with some breacher marines, and then someone will mention a spartan, then a xiphon, then you'll see the sicarian and then before you know it you've bought an imperial knight and accidentally spent a grand without realizing it!