2) NO you do not NEED a detailed scanner. You just need a basic scanner (that's default) and that's it. Everything else gets you more credits and less effort but you don't need it. People who say you NEED it are being snobby optimizers. You need an advanced scanner like you need a fancy car to get you to work/school.
3) Not really sure. I like the window better on the hauler and it's cheaper to buy, so you have money for extras and cheaper to fix. Adder has more equipment slots.
4) You are unlikely to see anyone past a few hundred light years, but it's up to you. Sell them and get some nicer thrusters, so you'll go faster if anyone comes after you.
Your point 2) is confusing to me as you have mixed the terminologies between two different scanners so I'll expand.
You don't need a Detailed Surface Scanner but it is recommended. This is used for flying up to each planetary body and doing a detailed scan of the surface. This increases the revenue you get on the return visit and is really what exploring the galaxy is all about.
I would say an Advanced Discovery Scanner is essential as this will identify all the planets in the system in one scan (infinite system range). If you use the basic disco scanner then you have a range of 500LS (nowhere near enough in my experience) and an intermediate disco scanner is 1000LS range (I think) which is still nowhere near enough to scan all astronomical bodies in a system.
If you are serious about exploring you need an Advanced Disco Scanner and a Detailed Surface Scanner. If you're not so serious but just want to dabble with getting your name on some stars then a basic scanner will do but remember you will only get your name on that one star in the system plus anything within 500LS if you fly out to scan them.
You also need to purchase the biggest class, most highly rated fuel scoop you can afford. Absolutely essential for exploring.