Is drupal appropriate for my web application?

Whilst cross posting is discouraged there is no clear answer to your second question there, which seems to be 'why use Drupal when you can build yourself:. As you say, with Drupal you gain login (and loads of other functionality ready-built, amounting to a huge and flexible eco-system, with in-built caching, security, failed login protection, a module to integrate with Solr Search, a contributed Search API, image handling, taxonomy (category tags), forum, polls, contributed GUI tools for building and styling HTML pages (blocks and tempting system, and in contributed moduldes Panels, Display suite, Context), a GUI tool for building database read queries using Views module, enterprise-grade eCommerce tools, and so on). If you the only part of the core and contributed code that you need is a login feature, Drupal is probably over-complex for your needs because this complexity does come with a cost in terms of maintenance. In addition, when starting with Drupal there is a temptation to use some of this stuff just because it is there, free, and easy to switch on, and this can create a site which takes a lot of maintenance and hosting, with features you may not really need.

I'm happy to respond to questions posted here, but I'm not going to reply to questions on another site. If you'd like to edit your original post to include the question, you'll be more likely to get responses.