Questions about what is possible: Requirements, multiple selections and auto calculations

Firstly, thanks for any help anyone is kind enough to give me. I'm a very amateur coder and mostly have experience with Hero Builder doing custom sheets. I'm currently making a sheet for my home brew game and I want to take my sheet to the next level. Before I go down the rabbit hole of trying to make this work, I'd like to know if it's possible.  My game features a large amount of abilities (over 450) based on skill levels. Is it possible to have a character sheet that can handle this amount of information as well as logic to account for what it is possible to select on drop-down menus. Does it require a compendium (which I have asked to be a volunteer for) or can I make the sheet access data outside roll20?  For an example of what I want: on the sheet you would select a skill and then select a level of that skill. After that you would be able to select abilities from that skill restricted by the level you chose. Then each ability would have a roll (which are very standard in my game) with a description of effect. I would also like to do something similar with equipment.  If it's possible I just need to be pointed in the right direction for tutorials and learning materials. If not I'll find something else to do with my free time.  Thanks in advance. 

Roll20 uses html, css, and for advanced features, javascript. Tutorials for those are everywhere, but the roll20 wiki has roll2o-specific help. Start with the page on Building Character Sheets. What system are you building a sheet for? Sheets can contain as many abilities as you want, but if you have lots of abilities in lots of sheets, it can cause lag in roll20 which is bad. Instead of having 450 abilities in the sheet, you can use repeating sections, and let players choose which abilities they want, keeping the number down. Repeating sections are a way to create stuff as you need them (like a table of skills, or an equipment list, etc.), where there is a masisve list of options, but players only have a small number of them.  For an example of what I want: on the sheet you would select a skill and then select a level of that skill. After that you would be able to select abilities from that skill restricted by the level you chose. Then each ability would have a roll (which are very standard in my game) with a description of effect. I would also like to do something similar with equipment.  There's several steps here and some of them are easy, others sound very tricky but it's hard to know without specific detail. Also, there are often multiple ways to do things, and if something looks like it cant be done, there are often other ways to approach it. This part is the tricky part: " After that you would be able to select abilities from that skill restricted by the level you chose."  I'm not sure you can get dynamically created lists to appear in a dropdown in roll20, but  if you can build, ahead of time, a separate list of abilities for each skill and level combination, and program that into the sheet, this is doable. 

I'm already using repeating lists for inputting abilities and equipment etc. It just requires the user to copy and paste descriptions from an excel sheet of abilities and make sure all their calculations are correct. I've figured out roll templates (and CSS) this week which has helped immensely. Knowing that what I want is possible with a lot of work just what I needed to know. The data entry will probably take longer than the initial coding. The system is one I've built from scratch. A skill based xp buy system with soft classes and very few limits on what you can do. The combat is very similar to 4e D&D in that everyone has lots of abilities and cool shit to do. The world it is in etc. is a bit of a secret until I have more finished. I'm jealously guarding some of my ideas. Thanks for your help.

Since its a homebrew system, you can probably hard code a lot of that excel data into javascript, and use sheet workers to autofill attributes with the relevant data. That will be easier on your players. Look up the "javascript object literal" or json data storage for ways to do this that are pretty efficient.  Once you understand that data model and how to use sheet workers for this, you could convert the data stored in excel into the correct format with a vba macro or just careful use of functions and copy /paste. I wouldn't worry about that yet though. better to concentrate on getting the sheet working, this is always something that can be added later. But we are here to help when you need it.

I found an Excel->JSON converter that got most of the work done for me. Thanks for the advice. Once I've finished cleaning it up, I'll have to figure out sheet workers. JSON seems pretty simple and I see it's used for compendiums as well so this is a good exercise for me. My character sheet is already working fine. I just want to make it more automated and easier for new players to understand.