There are a few more crucial items that no good tool box should go without. Remember to keep your stocks up on all the following must have supplies.
Tapes - $2
Tape is a necessary element in most mods with a huge selection on the market. A lot of people recommend 3M tapes as they are of a professional grade and do exactly what the package describes.
Masking tape is great for marking out cases with straight lines and screw holes. Low Tack tape is ideal for painting as it peals of very easily without damaging paint or other delicate surfaces.
Double Sided Mounting tape can be used for mounting components like water pumps, lights and can even be used to hold acrylic case windows panels in place. Electrical tape is handy when cable sleeving I use different colours to identify my wires and for taping off plugs for easier sleeving.
Glues - $5
Glue isn't the most commonly used adhesive when it comes to case modding and types will vary depending on what surfaces you are trying to stick. The most common type used is the glue gun - these are very cheap and refill sticks are also inexpensive. I also like to have good epoxy glue like araldite handy for stubborn glue jobs.
Sandpaper - $3 for a pack
Tasks like lapping CPU's and painting use a wide variety of different grit sandpapers from the very course 200 to the very light 2400 grit. It is also a good idea to have a selection of wet and dry sandpaper as different finishes requires different paper.
Cable Ties - $2
Cable ties are the ultimate tool in any modders arsenal allowing for mounting anything anywhere. Available in different colours, lengths and even reusable Velcro ties, the humble cable tie has come a long way. Easily the most essential tool for cable management allowing even the most experienced builder to get the ultimate clean and airflow friendly case.
In conclusion you don't need to spend thousands of dollars on the "best" tools.
In the past I have put off buying necessary tools and just getting by with what I had at hand, instead spending the money on other build components. But after two or three mods I quickly learnt that the proper tools are an investment, as chances are you will use the aforementioned tools for numerous projects not limited to just your modding hobbies.
Take your time, research tools and prices thoroughly before you purchase and in no time at all you will find yourself equipped for any job!
Mod now, sleep later.