Is $800 fair for a tweed Deluxe...that is a question only you can answer after playing one...I try to never look at prices of amps and relate that to wattage. It simply doesn't work that way. I happen to think that a 5E3 Deluxe is one of the greatest amps ever but guys that are after lots of clean headroom or lots of distortion would disagree.
FWIW, the difference in 12 watts and 15 watts is nothing you'd ever notice...you could easily run a tweed Deluxe with an AC-15 with no issues. You CAN NOT hear a watt, it simply can not be done.
If you want 5E3 tone a good clone is the way to go...an original late 50's Tweed Deluxe is around 3k+ these days, even the most high dollar clones out there are a good bit less than that.
On the topic of Celtic amps Scott owns Celtic amps and makes a few amps, the ones I have seen were less than impressive as far as the build quality and really left me wanting more in the tone department, I hate to say that as Scott seems like a stand up guy and I also know that there are several guys here that have and use Celtic amps but to me they are marginal amps built by a hobbiest and it shows.
Now, on the other hand Bruce Collins who owns and runs Mission amps and is one of the best builders and amp techs I've ever known. He built my Super, worked on my 62 Deluxe, 69 Twin Reverb and is soon to get a crack at my 60 model Ampeg Jet...he really is one of the best, right up there with guys like Gerald Weber (Kendrick Amps), Mark Sampson (Matchless, Bad Cat, Star) and Fred Taccone (Divided by 13). He does build clones of amps that seem to be in high demand but he also works on all amps from Crates to Dumbles and also designes his own amps as well.