Obviously 3-5 days is not much time. I think how much you "need" depends a bit on the sorts of things you like to do. Bangkok is an enormous city. While staying near BTS or MRT is a good idea, you can't get to the most important sites like the Grand Palace and Wat Pho via those types of public transport very easily...altho I suppose you could take the BTS to Saphan Taksin and jump on a river boat. I wouldn't do that without a good guide for your first trip. That said, taxis are plentiful and relatively cheap.

If you can swing 5 days I would suggest a day trip to get out of the city and see more of the "real' Thailand. A trip to Ayutthaya with a stop along the way at the palace at Bang Pa-In is a good trip. You see the ruins of Ayutthaya which was once the capital of the country before it was sacked by the Burmese.

Bangkok is full of nice hotels. The level of service is quite excellent in most of them. You may also want to consider a serviced apartment unless you need a full service hotel. I personally like having a kitchen and a bit more space.