Help putting porcelain tile?

Hello Everyone
I have a question i'm hoping you all can help me with. i have a large terrace/pool area which is about 2000 sf. right now, i have about 20 year old porcelain tile on there. it was well put and it has not cracked but it's just old and ugly. we've tried cleaning it and sealing it, but it's just really ugly, old and grey. so we've been shopping around the last few months and found a great deal on about 2500 sf of new outdoor non-slip 18x18 porcelain tile. so now my issue is, how to install? i've had about 5 installers come out and everyone gives me a different opinion:
1) first guy said he's lay it right on top of the current tile w "multi bond" and it would be just fine. 2) guy # 2 said we had to pull out all of the old tile that is there now bc the new tile will just come off in a few months or in a year if you lay it right on top of what is there. he says there is no way porcelain tile will absorb any thin set or bonding and that it will just start coming off, especially since it's outside and with the change in weather. i live in north florida, and it gets really hot, and in the winter it gets cold. 3) guy #3 said he would simple scratch it up with some tool so that it will absorb. no need to pull it all off. it's hard messy job to pull everything out and they want to do that just to charge more. he just scratches the bottom tile up really good and puts plenty of bonding and thin set on it and it should last another 20 years. 4) 4th guy said he uses this special new material called "PORCELAIN SET" and it the best. it's $14 a bag but this stuff will bond porcelain better than anything else, no need to scratch anything. before you think he wants to scam me, he doesn't sell it. they actually sell it in the store where i bought the tile. he says most folks just want to buy the regular multibond product they sell in home depot that's $6 a bag. but he says that he's noticed that this particular product that they sell in the specialty floor stores works really well. 5) last guy said the PORCELAIN SET stuff is just an excuse for you to pay more $ for bonding. he insists that regular multipurpose thin set and bonding will be just fine. he says it all depends on what is there now. since what is there now is in good shape and is not cracking or coming up, then laying new tile on top should be no problem. If we scratch it up and put in regular thin set and bonding, we should be just fine for at least another 12-14 years.
So you see, i've gotten a different story from everyone. they all add on another $1 a foot for pulling it out and charge about $1.75 to $2 to install it. so if i can save 2K and not have to pull it out, that would be the best. i'm 5 months pregnant, so this isn't the kind of job i can do myself right now. i need to find a good contractor at a decent price. i just want to know what you all think is the safest way to go. this is a big investment and i want it to last me a long time. i don't want it falling apart in a year. but on the other hard, i don't want to go crazy and pull out 2000 sf of tile when it's possible there are alternatives.
any ideas? thanks very much!!
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Try posting at www.johnbridge.com
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it would be best and have the most longevity to pull the old tile. porcelain tile is really hard, harder than other ceramics, so you might want to have them test the machine they're planning on using to scratch it up before accepting one of the other ideas of laying new tile on old.
a thinset, of any kind, will not bond well to a glazed tile. you haven't stated what the surface of the old tile is like.
it would take no more than 1/2 day for a crew to remove the old tile and grind down the old thinset. moving and getting rid of the old stuff is the expensive part.
tell 1 and 4 to go away.
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