There's a quiet confidence in knowing exactly what you don't know yet. That's where this reader starts, and it's the right place to begin. Preparing 7,000 rows of membership data for a system switch isn't a technical problem first. It's a clarity problem. The questions asked here, about merging by System ID, about whether the data volume is manageable, about when to clean, are the same ones every thoughtful person faces when the spreadsheet outgrows their comfort zone. The honest answer is that none of these steps require a computer science degree. They require a method, and a method is just a sequence of decisions made in the right order.
The simplest path forward is also the most direct one. Merging Sheet 2 and Sheet 3 into Sheet 1 by System ID is a lookup task, and for a beginner, the most accessible route is not Python or Power Query. It's the familiar VLOOKUP or INDEX/MATCH in Excel, even in the web version. Those functions work column by column, and they work fine at 7,000 rows. The fear that 20,000 rows might break something is understandable, but it's unfounded for a lookup. Spreadsheets handle millions of rows in a single column lookup without breaking a sweat. The real constraint isn't the tool's capacity. It's the discipline to check for duplicate System IDs before you start, because a lookup only returns the first match it finds. One duplicate and your credits and debits land on the wrong member. That's not a data problem. That's a trust problem.
On timing, the answer is to clean first, but only lightly. Don't attempt a full audit before merging. That's a trap. Instead, do a quick pass for obvious issues, duplicate IDs, blank required fields, inconsistent formatting in key columns like email or phone. Then merge. Then validate the merged result by spot-checking a handful of records against the original sheets. Why this order? Because cleaning before merging means you're guessing what matters without seeing the full picture. Cleaning after merging means you're working with a single, unified view, which makes errors more obvious. The user's instinct to ask this question is sound. The practical answer is that a two-hour pre-clean saves a full day of post-merge confusion, but a full pre-clean delays progress without adding proportional value.
The deadline of April 27 is tight but realistic. The best next step isn't another tutorial. It's a test run. Take twenty records from Sheet 1, merge the corresponding credits and debits from Sheets 2 and 3 using VLOOKUP, and verify the output. That small loop will reveal more about the process than any guide can. If the web version of Excel blocks a feature, don't fight it. Download a copy to the desktop app, do the work there, and re-upload. The tool should serve the task, not the other way around. This reader is asking the right questions, and that's the only technical skill that actually matters.