I'll let Joe speak about the prestone flush kit. I've been accused as of late for being jerk... so no comment. As far as using OEM or aftermarket parts... that has been a bit of a debate lately too. SOME aftermarket parts may function properly in the LS for quite some time... but when it comes to the cooling system,,, it becomes questionable. The V6 is possibly a little more tolerant... but the V8 is pretty much a FUBAR Jag design,,, so IMHO... OEM parts are better for long term reliability.
I'm sorry your wife is saying this is the last time. The LS is actually a pretty good vehicle,,, even with it's faults. Thing is... at about 100k is when things start to go wrong on ANY vehicle. Unfortunately,,, the parts for the LS are more cost prohibitive to do the proper maintenance. As the saying goes... "in for a penny,,, in for a pound". How fitting to use a British saying,,, for a British designed vehicle. As I've said before... the LS is "pay to play".
Hope you get the issues worked out on your LS,,, and that maybe the wife will let you keep it. Be ready for costly suspension repairs in the next 30k -50K miles. The rear lower control arms are an issue... and on the cheap side of OEM,,, about $500 for the pair... not including any labor. Whatever you do... don't buy the stamped steel/welded Dorman version of the rear lower control arms.
Not trying to get you discouraged. Just letting you know a bit of what's coming.