well, you can't sell all 60 gigs. You need room on that for your operating system, swap, etc... so you may need to adjust how much total space you have to sell. Unless of course your /home is mounted on a seperate 60gig drive.
I've never used ensim, but I believe 3.1 controls the resellers to what their allocation is. If you're going to have 60G, you'll probably have a dedicated server and will have control over allocations. Like I said, I've never used ensim, but I would imagine that if you controlled the box, you could just tell it that it has 120-180Gig and oversell that way. Also, you could always make an agreement with resellers that they could purchase a 1Gig plan that will charge them if they go over, and then allocate 2Gig-3Gig for them - if you can't tell the box you have more than 60Gig.
If you're just thinking about it, I would imagine then that you don't have a huge customer base yet. Why not go with a reseller account first until you have enough income to pay for the server?
Have you already gotten your UCG server? If not, check out http://rackforce.com , they are better suited for newer users. Their support is great.
Also, 8.0 is kind of new. You might want to consider using something prior, or waiting until 8.1 comes out.
Just make sure you provide your clients with some sort of backup. Or if something goes wrong, you can have all 200 clients coming right back at you, and they aren't going to say "Oh, it's okay... it happens".
IDE drives are cheap. Save yourself a headache now and just get two of them. Then run daily/weekly/monthly backups and store those on your spare HD. Drives do go bad. Its one of those things where people always think "oh, it can't happen to me"
personally if i have a 60 gb hdd i look at selling 30-35 gb of hosting on ensim max, once you have taken into account the op sys, the cp , email and databases etc etc etc you dont have 60 gb to sell, then theres the swap file, you never aim to sell 100% f your resources, its bad business, i sell about 60-70% and let people upgrade, leave some slack etc. if there is no space then move them to another server. by the time your server is 50% full you should be commissioning your next server good luck
oh and redhat 7.3 can be better then 8.0 7.3 is a refined version of 7.0 , 8.0 is a new start in a way so it will have bugs and problems, they rushed its release which never helps so sayng 8.1 is better then 7.3 could be true but 8.0 is a little to fresh to be pushed onto production servers as yet.