On your inital question: You can apply for a visitor visa with validities as long as 10 years (although individual visits are limited to a shorter duration). Note that while a visitor visa would be valid for attending an interview, you would have to convince the visa office that you intended to return home after the interview, then apply freshly for a working visa and work permit. You can read more about types of visa, and the rules for them, on the offical UK government website, gov.uk
However, various elements of your story suggests that there is no real chance of a job for you here, and that you would be denied a visa to the UK due to your confused evidence anyway. You mention that the documents you received contain a "job offer". In the UK, a "job offer" letter would take roughly this format:
Company Name would like to offically offer you the role of job title, with a salary of £XX,XXX per year (plus additional benefits, please see the company handbook). The intended start date of your emplyment would be dd/mm/yyyy. This offer is conditional on receipt of valid references and confirmation of your legal right to work in the UK.
If you have any queries, please contact contact name, email address, or please perform paperwork actions to confirm your acceptance of the offer.
The offer is received after all interviews etc have been performed and you have been selected as the/one of the candidates for the role. You would not receive an offer before an interview and if you were to include an offer letter in a visitor visa application, it would work against you, as it would be evidence that you intended to work and not leave the country, in opposition to the rules of a visitor visa.
On top of this, the situation with the travel agent is very strange. If a company is expecting you to travel across the world for an interview, then return, then travel again to relocate to the country, then one would normally expect them to bear the costs. There are employers who might not do this, but they would normally then not expect you to attend an in person interview. In the event that you were expected to fund the trip yourself, or even fund the trip yourself and later be reimbursed by the employer, then you would be free to make your own arrangements, and not simply approached for money by their travel agent.
The travel agent's statements and requirements are themselves highly suspect. The requirement for a bank transfer (internationally!) immediatly raises red flags. Bank transfers (along with Western Union, Moneygram, etc) have no procedures for refunds, meaning once the money leaves your account it is gone, even if you can prove it was sent to a fraudster. Any reputable (even small) UK travel agent would be capable of accepting card transactions.
The claim of having your visa (and other documents) delivered to you at the airport seems to be a way of making you not realise the nature of the scam until the last moment. Any real visa will require involvement from you in the process, inclduing a trip to a visa fascilitation centre. You won't be able to board a plane to the UK without having a valid visa (or visa-exempt passport), so if the airport you were meant to receive this at was in the UK, then that is too late.
One of the other answers suggests that you may also be approached for additional money to "ease" the way through UK beaurocracy and officals. You should know that this is not the way things work in the UK. Officals are sufficently well paid that they do not need a steady stream of small bribes to support themselves, and the penalties for being involved in such behaviour are massive. This is not to say that there is no corruption in the UK, but it is either more subtle and not relevant to this process, or considerably out of the price range of anyone looking for employment.