Tablets with keyboards cost almost as much as cheap laptops these days - that's one of those myths that just won't die. If you really want to save money, you can get a solid used laptop from the last couple of years for the same price, and it'll last you a long time. But whether tablet or laptop - both would work for your work tasks, the real question is how much flexibility you need.
What the others already mentioned about multitasking limitations is real. But honestly, people sometimes overestimate that. With Google Docs and emails, you don't really notice much difference in everyday use. The real problem comes more gradually: when you eventually need to quickly open an Excel file with a specific pivot table format, or need software that only exists for desktop, or just start noticing that handling it gets more and more fiddly. With a tablet you'll get frustrated.
My take: if you really only do these three things and that stays the same over the next two or three years, a tablet would be enough. But people change their requirements - either you suddenly need more, or you get annoyed by the little things. A cheap new laptop (or a good used one) just gives you more room to breathe and doesn't necessarily cost more in the end.