The problem is, by the time you buy the adapter, you might as well get a different head unit or amp. That line output converter alone is what $60-$100? There's your head unit. I have no clue about what your factory head unit is. If it has GPS/nav, then you can't replace that with a $100 head unit. Just about every other feature you can.
Why would an aftermarket head unit overheat? That just makes no sense at all.
I would start fresh too (if you don't currently have nav).
I love
Crutchfield, I don't think anyone can say they aren't the best (car) audio website there is. If there is a top 5 list, I think this would also be on it. Here is the SALES page:
WoofersEtc.com - The Online Car Audio Superstore, the Largest Selection at the Lowest Prices!
www.woofersetc.com
2). You started talking about Morel and then listed kickers and the cheapest soundstream amp you could find. Did your budget change from the first post to the second?
I would also suggest, you figure out your ENTIRE SYSTEM before you buy anything. Especially with a remanufactured amp, that you won't be able to return, or have a warranty on. Make sure it will all work together. A good deal on one, might force you to buy something you don't want so that it will play well together. You might change your mind, and you should consider your options, like: is it worth it to get a 5 ch amp and add a sub if the numbers work, if thats what you want. Maybe it's worth it to get a 6 ch amp for your setup.
I would be hesitant about a 10awg amp kit. You can go as big as you want with wiring. It only costs a little more, and takes up a little more space. The KICK YOURSELF part is buying a 10awg kit, then upgrading and now buying a 8awg kit, or a 4awg kit. Wiring is something that personally, you should upgrade to "as big as you might ever need", the first time. You're already spending a lot, but it's $20 now or another $100 later. Your call, just sayin.
Personally I wouldn't even bother with a 90w x 4 amp. You can buy head units that will go 50w x 4. You can get good sounding 6.5" on 50watts, it kind of depends on what YOUR BUDGET IS, what you listen to, and how loud you like it. Not buying an amp, and kit could make a difference in what your options are. Heck there's a couple 60w focal 6.5s in that link, that you could play off a 50wx4 head unit. Be all in for about $350 on two pairs of those and a head unit, and call it a day.