The modern cruise ships are 'stabilised' & sea sickness is a rare event on this type of vessel. Your going on a cruise ship, not a north atlantic trawler! Go to the websites of any cruise line & you will normally find pictures of cabins. Choose Mid ship & lower decks for less movement. As for diesel smells, vibrations etc. that you may experience on a car ferry, forget it on the new superliners, they are floating hotel resorts. As for your last question like anything in life you get what you pay for, & be aware that some of the 'cheapies', in some cases, are converted car ferry's & ex container ships. Talk to a cruise specialist, tell them your expectations, ask them how many & what cruises they've been on. The freephone number at the top right of this page might be a good starting point. Good Luck!
If you have never cruised before and are a bit unsure of how your body will deal with the expecience then my advice would be, as previously advised by WILBA, choose a cabin on the lowest accommodation deck available which is also half way between the front and the back of the ship. This will ensure that you are not going up and down as if you were in a lift should the weather get rough! Choose an outside cabin with a window if you feel that you may become a bit claustrophobic, but don't loose sight of the fact that most of the time spent in your cabin will be sleeping where a window won't really make much difference! If you pick one of the many modern ships to cruise on (built within the last 10 years) then fuel smells will not bother you. Once again, vibration is usually associated with older ships. Just one very important thing to bare in mind when making the final choice of cabin if you want to avoid excess noise. Don't choose one that is right next to a stair well, launderette, or above/below a busy public area such as the nightclub or showlounge,etc. If at any time you do feel a bit lightheaded, then DON'T shut yourself away in your cabin! Get out on deck, take a walk around, get some fresh air, and you will adjust to any movement of the ship a lot easier. Happy cruising.
Agree with all thats been said and will add go on a short cruise first if you don't like it its not for long and if you do you can book another?I am a bit claustrophobic and have not had a problem your cabin will be similar to a hotel room or a caravan enjoy it?