I don't have a MEP802 or MEP803 but I do have some thoughts and experience on a "spares strategy"
Your on the right track stocking the "obvious spares". No harm is doing that. But...how far you go in stocking spares is dependent on how badly your immediate needs are for maintaining power and you local Power Company's outage frequency. My local power provider is hit & miss due to so many tall trees next to power lines. Couple that with those of a "Prepper Mentality" of conceivable long term partial or total grid failure (increased potential for massive fires, earthquakes, storms or anything else in the realm of a potential area grid failure) and your talking about having back up redundancy. The "two is one, one is none" mantra is wisely stated when it comes to "mission critical" equipment.
If your situation is mission critical like mine is with wells, pumped septic, freezers, etc. you might also consider taking a "long term view" and start scouting for a spare genset before your model genset becomes harder to find with eventual scarcity of parts and ever increasing prices. If you can take your time now to locate a reasonably priced non-runner that you can repair it will prove to be money well spent. Craigslist worked for me when I started my "spare unit search". The guy I bought my 2nd unit from gave up on repairs and it only took me a day to get it running with no parts required!
I had a genset fail halfway into a week long outage and troubleshooting and repairing late at night in the dark icy cold just wasn't practical. It was really nice to be able to roll in backup genset #2 during a cold stormy night when needed. Even if I had found the problem soon after my genset failed, I wouldn't have had the part I needed to effect repairs even though I had stocked the obvious spares, as a control transformer had failed.
When you are able to roll in a "functioning spare unit" it gives you the time to thoroughly troubleshoot your primary genset. Also I might add, without the added pressure of family incessantly asking how much longer till power is restored. Possibly facing having to go to bed in a cold dark, or hot, house and waking up to an unknown situation & facing a parts sourcing dilemma. Having a backup unit at-the-ready gives you the the time to thoroughly troubleshoot the problem and then locate the exact part you need & order it to effect the right repair(s) as time permits. Once parts are on order you can then go in and watch TV & drink a beer, or whatever your fancy is, and leisurely wait for parts to arrive with no downtime!
I know it sounds like a luxury and too expensive to consider a back-up unit in addition to having a primary unit, however when you need it YOU REALLY NEED IT, and its worth every $ you paid and then some!
However, everyone's situation is different and each of us have to make a call on our best spares strategy.