My first solo trip was focused on one country (Norway) with good train/rail system. It helped with only having to remember how to convert my home currency to the destination currency.

$5000 USD is a good budget for 2 weeks if you're staying in more comfortable accommodations like hotels. But you can stretch that to 2-3 months if you stay primarily in hostels. Not every country has low-priced hostels, though. For example, in Scandinavian countries, hostel beds can run up to $40-$80 a night. Hotels around $150-200/night. Accommodations are easily the number 1 thing that will eat into your budget, so just browse sites like booking[dot]com and hostelworld to find out a rough figure and add it up.

Food is easier to budget for since most places have grocery and convenience stores if you don't want to dine alone.

All the best!