The 5 year test applies to surface use tanks only i.e. Tanks that have a gauge as an integral part of the valve assembly.
Tanks used for diving have to be tested every 2 1/2 years and full hydrostatic test every 5 years.
Back to your query, there's no real time frame on tanks either surface use only or ones meant for diving.
My tank for example is used both for diving and my guns. I've had this tank since the 90's and it has only had one valve assembly in all that time. Also it is a steel tank and has passed all tests in that time apart from the valve. So no signs of any rust inside.
Tanks also come in aluminum , which makes them a tad lighter and of course no rust issues. One issue with ali tanks , can be and I stress can be that the thread in the neck can 'stretch' over time , this will show up on the test though and usually means game over for that tank.
Don't panic though this is NOT common .
You should be OK buying secondhand as long as the tank is 'In test' and all tanks have to have the test data on them showing how long they have left before testing is required. If a tank is 'Out of test' any reputable filling station will refuse to fill until a test has been done and passed.
Also notice that you have no car ? So may want to consider a 7ltr 300 bar tank as they are a lot lighter to carry around downside will be if you shoot a lot then more trips to refill unless you get a bigger tank as well to de-can't from, think this was mentioned in an earlier post by RobF ?.
Hope this info helps.