A. Do I need to have HP/Complex time before getting my Multi, or is it do-able just to get the HP/Complex while doing the Multi?
Need? No. But, it's a really good idea to have some complex time and preferably the endorsement and some proficiency in a complex airplanes. It generally costs a LOT of money to rent a twin, so it's better to get used to as many new things as you can BEFORE you get in the twin. In fact, if it's possible, rent the single-engine version of the twin you want to fly first to minimize the learning curve. Most twins have a single-engine counterpart: Seminole/Arrow IV, Seneca/Lance, Baron/Bonanza, etc.
B. About how much does it cost to get the PP-AMEL Rating?
Cost depends a lot. Kent got his from a guy who churns them out in Michigan, and I seem to recall it was pretty economical.
Yep - This has got to be the low end right now. It was $1400 plus the checkride ($300 IIRC), and was about 7 hours. I did it with Tom Brady (dba Traverse Air) in Cadillac, MI. Tom specializes in doing multi and seaplane ratings, and he owns an Apache for the multi and a Clipper for the seaplane. It took 3 days, and followed a pretty simple formula. You are expected to know your stuff, and Tom will send you some materials about multi-engine flying in general and the Apache in particular for you to study.
At the other end of the spectrum, some twins like our local Seneca rent for $300/hr and you generally have to pay at least $50/hr for an MEI.
If you don't get it done in a fairly short time frame, you'll end up spending even more money because it'll take you more hours. If I had done my rating at my home drome, it would have cost me AT LEAST DOUBLE what it did with Tom.
However, from a cost perspective I would recommend waiting until you pass 250 hours so you can get your CP-AMEL rather than PP-AMEL. Otherwise you'll need to take an additional checkride to upgrade you PP-ASEL to a CP-ASEL and then upgrade your PP-AMEL to a CP-AMEL.
What Ted said. I decided to get my commercial done before adding on any additional categories/classes so that I can add them all at the commercial level. You just never know when it's going to come in handy to be commercially rated for whatever - I got to fly right seat in a Hawker 800 a few months ago just by chance! So, unless you have money to burn, get the multi done at the commercial level.