More ideas on how I could actually make it would be helpful, especially because most likely I am up for the challenge.
I hate to ask, but did you do the search? The first part of being up to a challenge is doing the research....
Not trying to be rude here, so I apologize if it sounds like it - I am trying to be a bit practical and sincerely...there are already SEVERAL good manta scratch build threads out there - some here, some on warseer and some on advanced tau tactia.
A lot of people took the time to document their work - and just 30 seconds in google turns up many of them...
From the very very simple:
http://gravengames.co.uk/scratchbuilt-tau-manta/
to the very complex:
http://www.warseer.com/forums/showthread.php?158810-Scratchbulding-a-40K-manta-vs-thunderhawk-diorama
to one even right here, on dakka with amazing paintjobs.
http://www.dakkadakka.com/dakkaforum/posts/list/254689.page
All three of these feature very different design approaches - and none of them are "better" than the others - you will have to do research on them, and then assess your;
* design
Is it going to be solid? Will the top open? The bottom? The ramp come out?
Will your gun turrets rotate? does that matter to you? Will it be one piece, or maybe two for transport? What kind of a stand do you want?
Do you have to transport it short distances where it can sit in a trunk (a very big trunk) or rest it on its side in a back seat? (it won't fit in the front seat of most vehicles - literally).
Whats the max weight you want to comfortably lift by yourself (I just finished a thunderhawk a while ago - the guy I made it for - he is scared to carry it as he has a damaged wrist) - depending on what method you choose it could be very heavy.
* access to materials
For example, do you like working with foam and foam board? Can you get it cheaply in your area. How thick? (they don't sell thick insulation foam at many places in warmer climates for example).
Can you use bondo or filler? Do you have a place to sand it? Can you find oven bake clay in your area? Do you have an oven to bake it in that won't get you in trouble?
Would you buy bits to make the sixteen long barrelled burst cannons? Or would you try to scratch build them?
* skills in crafting different said materials
Have you used foam before? Do you have a respirator for sanding fine materials? Do you have an area with really good ventilation so you don't need one?
Are you decent at sculpting and working with oven bake clay? Do you have the patience for it? (I don't).
* your tools and work area?
Do you have a dedicated project area where you can leave out a HUGE model for possibly a month or more?
(i.e. not a dining room table).
Proper tools (cutters, sanders, straight edges, epoxy putty, large quantities of gesso for priming, etc.) and other supplies?
My comments here are NOT meant to discourage you, or anyway negative - these are just some of the things you will have to answer - above and beyond how to build it - regarding the types of decisions you will have to make.
I think your enthusiasm for tau and scratchbuilding is great - I honestly do - I LOVE crafting stuff - the manta is a very ambitious project.
Finally, play a game with a proxy of the correct size (cut out a shape in cardboard) - see if you even think you want one after that - if you do, go for it!!!
Take a look at those threads and others - and then once your start your manta build, create a thread (or a blog) so others can see your progress on your design.
best of luck and keep us posted!