If I were you.
Here's what I wouldn't do:
1.) Buy more than 1 album at a time.
2.) Start with BWP.
3.) Think that all Opeth albums are like Deliverance (in actuality they are all probably better)... I love them all and you really can't go wrong with any of them.
Here's what I would do:
1.) Prepare to devote a lot of time listening to each album before buying another one... plan 1 month for each before moving on. I realize this seems extreme, however, I think it's well worth it. You'll get a much better feeling for the band than most of the fans who jumped on the bandwagon after BWP and bought all the albums at one time.
2.) Don't start with BWP. If you do, odds are you won't appreciate the earlier offerings as much. BWP is very good with fantastic production and overall the songs are incredibly rich. The earlier stuff isn't quite like it, however the earlier stuff is actually more interesting (at least to me). It's kind of like comparing 80's metal and progressive-70's rock... both are good, however, the 70's stuff requires more listening to really get a feel for it.
3.) Here's the order I would do and why:
A. Morningrise - Simply because it's the hardest Opeth album to really get into. However, it's also the best once you do.
Really fantastic acoustic melodies and classic riffs that are the best Opeth has done..
B. MAYH - this will give you a feel for Opeth at their heaviest. Sort of like Deliverance, except way colder and I feel I should almost say, it's a more serious effort on the part of Mike and crew... wonderful vocals all around on this one!
C. toss up between Still Life and Orchid - if you really love Morningrise, then get Orchid... otherwise get Still Life - Still Life tells an awesome story (concept cd, so is MAYH btw...). On SL, Opeth puts it all together, beautiful arrangements, fantastic lyrics, Mike's voice is at it's very best and the production is the best of any Opeth cd. Yes, I love Still Life...
D. BWP - sort of like Still Life... some really fantastic parts to the album, however there are also some very boring parts as well...
a great album nonetheless, but the last one you should get.
Well, that's my opinion anyway... there are a lot of other good ideas here as well. Good luck. Enjoy!