Just select the part of the timeline that shows bad prview and go to the menu: TOOLS-SELECTIVELY PRERENDER VIDEO (make sure that RENSER LOOP REGION ONLY is selected!)
Custom render means that you can manually optimize your output rendering to the needs of your project.
About buying Vegas: if you manage to find somewhere on the internet the possibility to buy very cheap a previous version of Vegas (Movie Studio), you will only need to upgrade, which is much cheaper.
A script is a kind of a program. It does not add extra functions to Vegas but it makes live much easier, especially when you have to repeat some functions very often.
You can download free scripts from the internet or you can purchase e.g. the ULtimate S scripts (www.vasst.com) or the Excalibur scripts (https://www.jetdv.com/excalibur/home.php).
Both contain a lot of scripts, which can be started from Vegas. Especially the Excalibur scripts are easy to work with since they are accessible through the tabs in Vegas. Just check te websites to learn about all the possibilities.
And ofcourse, when you have a background in programming, it might be possible for you to write your own scripts…
Since you are just starting with video editing, you might consider to purchase Vegas Movie Studio Platinum instead of Vegas 8 Pro. This is much much cheaper and has a lot of the possibilities of Vegas pro. By the time you get addicted to editing, you can simply upgrade to Vegas pro 8 (or maybe version 9 by then…)
Vegas 8b is the latest update version of Vegas 8 pro. So you don’t have to choose between 8, 8a or 8b. You will always have the latest version.
If you own already a previous version of Vegas or Vegas movie Studio, you can purchase Vegas 8 pro cheaper (see the Sony website: https://www.sonycreativesoftware.com/buy/upgrades
You can NOT purchase Vegas 8 pro without DVD Architect. However, you will need DVD Architect to put your videos on DVD.
A second harddisk in your PC is highly recommended, although I know some people who use a laptop with an external drive. But normally, an external drive is used for storage and back-up while the internal second drive is used for the footage in your project.
Since your footage is already DV-avi, there is no need to use any kind of convertor. Just connect your camera with your PC via a firewire cable. Most likely will that solve your problems