I work for the RSPCA so I do know what I am talking about and I would ask that you PLEASE take on board what I am going to say....
Before taking on a pup while you work full time ask yourself these questions
1 How are you going to housetrain the pup? They have to go at least every two hours and every time they wake up.
2 How are you going to feel when the pup is destroying furniture and being absolutely hyper because of lack of stimulation and boredom during the day?
3 How are you going to manage meal times which should be small meals between 4 and 6 times a day depending on the age of the pup?
And the answer to 1 and 2 is NOT as some people think

to confine the pup to a crate or locked in one room all day this only makes the situation much worse.
OK you stated a colleague has a dog left alone all day and its perfectly happy. First of all a pup and an adult dog have totally different needs and secondly just as all kids and adults are individual so are dogs. Just because Dog A has no problem being alone all day Dog B could pine for human company and hate every minute of being alone.
There is a very good reason why the RSPCA and the majority of animal shelters/rescues will not rehome where the pup is left at home all day. We understand they are cute and adorable and you really really want one but sometimes you have to think of the animals welfare the kindest thing for the pup.
Please do reconsider, either take on an older rescue dog or wait until the school holidays so you can spend time getting the pup to settle and work on housetraining etc. There are too many young dogs bounced back to rescues because their owners have given up on them and they cant cope with the "behavioural issues" that 9 times out of 10 are brought on by the poor animal being left to its own devices at an early age and not having enough imput from the owner