Nesting VLOOKUP In IF/vice Versa & Pivot Table

I've attached a sample/equivalent workbook of what I'm working on which will hopefully make it clear(er).

>There are two worksheets/month. Both worksheets (represent 2 different categories) are structured the same, two columns: model code & $ amount. >The model codes change (in # and actual model), between categories and month.

>The data for each month rolls up into a year-to-date summary worksheet, with 4 columns: Model (includes all models YTD, each only listed once), category1 YTD, category 2 YTD, & Total YTD).

Previously this had been done by manually entering any new models for the month into the rows in the YTD summary sheet. And the totals for each model (highlighted in yellow in the YTD tab in my sample) were just done by an adding formula, with the new month's data manually entered into each individual cell at the end of the formula (...+X). I know there's a much better way to do/automate this! (there are a lot more models than I've put in my sample aka it's way too time consuming manually).

My problem is twofold:
1. (main issue) I have been trying to do this using various IF statements nested in VLOOKUPS, and vice versa, but the issue that arises is for models in the summary sheet that don't exist in a given (month's) table. I want the value for those models (for that specific month) to be zero, but I cannot figure out how to get that to work in my formula. The only piece that works for me thus far is =VLOOKUP(A3, 'Jan Cat1'!A2:B18, 2, FALSE), but I've tried nesting it in IF statements, nesting IF statements in it, using ANDs & ORs, no avail.

I'm not even sure any of these options are the best ways to reach what I'm ultimately trying to do. A pivot table may be better? But I will need to keep/preserve the summary sheet for each month (so there cannot just be one big updated master pivot table).

2. If I could find a way to automate/refresh & update the row of models each month, it would be the sprinkles on the icing of this cupcake.