to start your multi engine training you need to have 70 hours PIC. most people rack this kind of experience up at around 100-120 hour mark, so it could be used to help hour build. having said that, it is a big step up in power/performance/speed and don't expect it to be a doodle. personally, i think the multi rating should be attempted once you are familiar with complex singles (ie wobbly prop and dangly wheels).
all the prices you quoted are minimums. most people are not perfect and take more time than the bare minimum. factor in a contingency into your budget of about 20-30%. also, you have to also include the caa fees. believe me, these are quite steep. last time i looked, it was £607 to sit a cpl, £607 to sit an IR. plus to have you licence issued costs somewhere around about the 300 quid mark. the atpl exams cost £52 each, and there are 14 of them. in all you normally shell out over £2000 to the caa through your training.
working with just a plain cpl limits you to aerial work like photography, banner towing, parachute bombing. these jobs are like hens teeth so most people bite the bullet and get an flight instructors rating (circa £6000). this doesn't pay very well, but if you can live on baked beans then it is a very good way of building experience and getting into the industry.
i am not trying to put you off, but there are many pitfalls and hidden extras in this industry that people tend to overlook when deciding on a budget.