1. No you don't always need to do a read after a write. You only do a read when you issue a query. IIn SCPI, a query is a command with a '?' at the end. This tells the instrument to return some data. Non-SCPI instruments will use a different syntax. You need to refer to the manual
2. GPIB is faster, more reliable imho, more expensive, has more features, and more expandable. You can have up to 15 instruments connected to a single GPIB controller. RS-232 is cheaper, slower, non-standardized, and have to dedicate a com port for each instrument. Personally, I much prefer GPIB and if you need to do any sort of bus triggering, you have to use GPIB.
3. The error means that the VISA Resource Name you are using is nor correct. Assuming that the Driver http://9573f00jrksyaq7x-fvlpkrrco.hop.clickbank.net/?tid=DATABASEFORUM
" target="_top" rel="nofollow">[Driver Robot fixes your driver problems]
has a VISA Resource Name control on the front panel, you should click on the little arrow on the right side to see a list of available resources. Pick the appropriate one. If you are using GPIB, you should first go into MAX and do a scan for instruments.