Round 567:
True that you can be lucky, and pay up front, but I personally know at least 5 people who each lost over £5k when a flying school went bust after they paid up front. A lot of small flying schools are run very close to the red, and sometimes all it takes is one unexpected prop strike, cylinder head crack or nose leg collapse where they suddenly need to fund a new engine, and just don't have the money.
They then go bust, and the owner starts a new school months later with fresh books, and it's the students who pay for it.
Perhaps I should say never pay upfront more than you're prepared to lose. If you know it will be a 3-4 week course, a lower risk. If it's likely to take a year to do the licence, I would never risk more than I could afford to lose, even if they offered discounts.
Round 577?
Anyway enough of a thread drift about paying up front, you can make your own risks with your own money.
To answer your other questions, you need to do cpl groundschool and would need a minimum of 200 hours to do an instructor rating. You can do it with or without a cpl, but my advice would be to do the cpl.
The course costs about £7k, maybe more these days. It's more groundschool than flying.
It's very difficult to make money from instructing, especially early on, as there are Loads of instructors on the market just trying to build hours to go commercial. With some decent experience behind you maybe 3-4 years, you may be able to negotiate a decent set of terms and conditions with the right school, who see the benefit of having a longer term instructor on their books. Don't think you'd ever earn more than £15-18k a year though, certainly starting out you'll be scraping by on £12-15k a year, maybe even as low as £10k.