These are all questions to ask of your Instructor, however:
1. ......can I simply hire and build hours or still need to hire at the price of instruction rate even when the instructor is not with me
It depends on the pricing structure of the school but, because you are not Licensed, an Instructor will always have to authorise your flight (and presumably also brief and debrief you) as well as supervising you in some manner - it is therefore most common (and not unreasonable) for these flights to be charged at the 'Dual' rate, rather than the 'Solo Hire' rate.
2. .....most places qoute on hourly cost , so considering a £160 per hour for a c172 , if hired for 24 hours the cost will be significant while you may be only flying for 1/2 hours
It depends on what (agreed) terms an organisation hires you an aircraft. Most charge by the 'Flying Hour', but would likely impose a minimum daily charge if you were taking their (money earning) aircraft away for several days.
3.when you land on other airports, do you have to give them advance notifications or do you just enter the space and request landing ?
It depends on individual airports/airfields - most are listed as PPR (Prior Permission Required) and many specify that this must have been obtained by telephone (which, effectively, means before you Take Off).
4.if anything goes wrong with the plane while you are away from the rental base, whose resposibility is it to get it back? for instance technical failure that costs 2k , i shouldn't be responsibile , should I?
It depends on what terms you have agreed to hire the aircraft. An aircraft abandoned because of weather would always be the hirers responsibility; technical problem (did the hirer cause it? was it poor maintenance? or was it just bad luck?). I know one organisation that, for foreign trips, requires the renter to agree to pay up to £10,000 if required to recover the aircraft.
5.when doing solo , can you take a friend with you or should be on your own ?
As a Student you do not hold the qualifications entitling you to be PIC in an aircraft; so you are not entitled to take passengers and can only fly on your own and then only when authorised to so do by an Instructor.
Interestingly, due to this question, I looked up 'solo' in the OED and the definitions given were not very useful (it did not say 'on your own' which I expected)
Of more use is the EASA definition:
‘Solo flight time’ means flight time during which a student pilot is the sole occupant of an aircraft.