I spent 6 months in Cardiff last year for work and found it a lovely city and growing. As for travel from London to Cardiff (don't pay the extra to fly into Cardiff) you can easily take the train - from LHR, take the Heathrow Express (or tube if you really want to save money and don't mind the time) to Paddington then hop on the train to Cardiff Central from there. The ride is about 2-2 1/2 hours. Weekends and off-peak hours are cheaper. Or, as the OP mentioned, coach travel is even cheaper and you may be able to catch something right from LHR. Bristol is right close by that offers many more flights than Cardiff.
If you like outdoors life, Cardiff is the place to be. The Brecon Beacons offer great hiking, as does the Pembrokeshire coast. The Gower offers surfing (wear a wet suit) and pretty beaches
For travel around the UK, you can use coach or train for bigger cities and locations. Car hire is the best for out of the way places. But remember that petrol is about 1GBP/litre
The working situation is more difficult. There are several agencies in Cardiff. I know there are 3 located right near the Castle at the top of St Mary's Street and on the corner of Queen St.
would you like to offer any special advice to a first time traveller?
Be open-minded and try to explore to the best of your ability. I enjoy getting a local paper to see what's going on.