[Nyororin]

First, there are a few things to be aware of.

Assuming that you DO sit for and pass the test - the 一級 alone says little about your language skill other than that you could pass the test. It is usually a bonus to have, but it doesn`t signal that you have true skill at the language. There are countless people out there who study for the test, pass the test, but who can`t speak Japanese at even an elementary level. It has gotten a little bit better in recent years as they`ve changed the format for some parts of the test, but there is only so far you can go with a non-interactive test.

I recognize that you have some level of language skill, and that as you`ll be studying in Japan you will likely be able to hold your own after the course...
However, with just the 一級, you`ll be in the same market as everyone else who studied for the test and managed to pass.
You are going to need something distinguishing you other than the JLPT. Basically, some other skills. It will be very difficult, if not near impossible, to find any type of job with just the JLPT and no degree / experience / special skills.

My advice - after getting the JLPT1, apply for normal university in Japan - and attend. It will improve your language skills, interaction, and get you a visa to stay in Japan for the length of the education... And the bonus of a degree at the end. Once you have the JLPT1 in hand, you should be able to apply to any university (not just those with international courses).