I'm sure your employer loves you posting his cost on equipment
If you are happy shelling out $400 for a 21 watt HID, consider purchasing a 10 watt and putting the extra money towards other gear, or higher quality instruction. You do get what you pay for in cave instruction.
I know I was freezing in a wetsuit at the Intro level, and moving into a drysuit was the best thing I ever did for my diving. They can be had cheap on the used market sometimes, but as with all used gear, be prepared to wait for a great deal.
I began Intro with doubled al100's, and they worked well for me. But, 95's were a great leap forwards. With practice, you can see a lot of stuff on 6ths of 95's, at one point I was managing 1500 foot swims in Ginnie on 6ths of 95's. Moving up to lp120's gave me egads of gas.
When I moved into sidemount, I used the lp120's for a bit, but they are awful heavy no matter what, so when a buddy offered to let me use his hp120's in exchange for me letting him use the lp120s, for trimix dives, I jumped on the offer. The hp120's are great for me in sm. I've also used 85's for some sidemount stuff where I didn't need all the gas, and didn't want to carry even the hp120's (in Florida summers, hiking gear through the woods, and in some cases using ropes to get gear into a sinkhole, I welcomed the smaller tanks with open arms) I recently picked up some 95's for backmount diving and still love them.
In short, I feel there is no perfect tank, and I can be happy diving nearly any cylinder. 95's are common and you can find good deals on them used. I would not take intro in a single cylinder, as I do not feel that single cylinders are ideal for cave diving. But, if you aren't man enough (or woman enough) to stick to your limit of 6ths in doubles, (honestly, the instructors that are worth their fee would probably be able to assess that you are not prepared mentally and would not pass you, but plenty of people do get passed without the maturity for this sport) then don't go with doubles.
Lots of people jumping on the sidemount craze. It's super popular nowadays. Lots of people with lots of arguments for or against. Some even argue that no one should dive backmount and everyone should start in sidemount. Obviously, I dive sidemount. However, I also dive backmount. Each has their purpose. Don't go sidemount because it's cool and trendy.
How new are you to diving? Not that I really care, it's more of a rhetorical question, but I would really, really, really urge you to find a good instructor, someone who you know will flunk you if you aren't safe. I've seen lots of new divers who aren't ready for overhead get passed by an instructor. If you get passed and didn't deserve to, you'll get a reputation pretty quick, it just takes a few people to see you in the water for word to get around that you aren't a good dive buddy. Much harder to fix that reputation than it is to simply accept that you aren't ready, and practice, go back and finish the class and pass. Much more important to pick a good instructor than to worry too much about what's the best tank for intro
Tanks are easier to replace than bad instruction.
[edit] Babyduck made a point I forgot to address, adding new gear before a class. Some are vehemently against it. I think it depends on the diver. If you have a good instructor, they'll help you assess if you are good enough. Ask if you can do a "get to know me" dive with them in open water, for them to see your skills, and for you to see their style of teaching and get feedback on things you might want to work on before the class. I know some teachers who require that sort of dive before they will do a class with you, and those that won't would probably be more than willing.
