I have had Hardi sprayers for over 20 years.  I don't think there is another pull type boom on the market that compares to their best booms.  You said it is an 8 year old sprayer.  If it is a Commander series it should be great.  There were a few years some time back that the Navigators had issues.  I don't remember exactly, but someone on this site should know.

Yes, the main parts are metric, but the fill valves have been banjo for a while.  As for metric parts, a good rubber and gasket supplier should have most metric o-rings and washers.  As mentioned, I would keep a supply of triplets and other fittings that might be prone to be hit on the boom.  You can also order parts from  a dealer that Hardi will ship direct.  This is a major improvement from Hardi and with a good dealer, even several hours away, you can get parts pretty quickly.   For tips, Hardi makes caps that allow you to use TeeJet or other types of tips.  I prefer Hardi's syntel tips (type of poly) to most companies stainless because they last at least as long.  The mini drift is a fantastic AI tip in that it isn't too course like some AIs, but still is a great drift tip for herbicides.  If you apply liquid N, the quintastream tips are somewhat resistant to height for even application and a far better than any 3 or 7 hole cap on the market for even application.  If you check the Hardi tip web site you will see how they work.  I would run the quintastream on a sprayer for N even if it wasn't a Hardi sprayer.

I like the diaphragm pump if you maintain it.  I change diaphragms and valves about every 20-30,000 acres.  This pump will offer some advantages such as self priming and makes flow changes more constant with changing ground speed as long as you use the throttle to change speeds.  You could get by without a rate controller.

Some parts are pricier and the original hydraulic hoses aren't always as good of a quality as U.S. companies, but later models are better.  Same with some supply hoses, but boom fittings and hoses are good.

If you listed the specific sprayer you are looking at, it would be easier to know what to look for.  Eagle booms are great, Force booms are better yet.