Here are some original old school D&D modules that should be easy to find and not cost you an arm or a leg if you buy them online:
I1 The Forbidden City (TSR, 1980): Old school AD&D sandbox setting in a "lost" jungle city inhabited by snakemen (yuan ti), frogmen (bullywugs), and lots of other weird and dangerous creatures. Lots of room for development here and no two adventures will play alike (due to multiple adventure paths and foes).
B4 The Lost City (TSR): This old school original D&D module consists of a pyramid buried in the desert sands which holds various degenerate groups battling each other for control of the underground empire.
X1 Isle of Dread (TSR): Lost island sandbox setting with lots of different adventure ideas
WG4 Lost Temple of Tharizdun (TSR): written by the master himself, EGG, this AD&D adventure takes place in a long abandoned temple to a dark god hidden in a mountain valley. Initial conflict is against monsters who now inhabit the structure, but if the party delves deep enough they will run into Things Best Left Undisturbed.....!
Axe of the Dwarvish Lords (TSR): This AD&D supermodule written in the 90s takes place in an long abandoned Dwarvish citadel now inhabited by an army of goblins.
Gates of Firestorm Peak (TSR): This 90s AD&D module takes place in a weird, otherworldly mountain that has connections to a "Far Realm" of madness and insanity. Sounds like it would fit right in with Raggi's stuff!
Not as easy to acquire are the classic Judges Guild modules Caverns of Thracia and Dark Tower (however, the 3.5 reprints should be a lot easier to find and are backwards compatible). Both are great and concern ancient, underground empires with lots of evil things lurking about.
I'm pretty much only familiar with BRP through Call of Cthulhu, and I don't actually own a copy of The Laundry yet, but I'll give a quick run-down of what I've been able to dig up.
Charisma stat replaces Appearance
No Magic Points
Personality Type is selected during
character creation, adding an
additional 20 skill points which are distributed
between specific personality-appropriate skills.
Personality choices include:
- Bruiser
- Leader
- Master (as in craftsman)
- Nutter (also starts with an insanity)
- Slacker
- Thinker
Laundry Assignment/Training is
selected during character creation.
Grants +10 bonus to several skills.
(This is in addition to Call of Cthulhu-style professions.)
Choices include:
- Archives
- Computational Demonology
- Contracts and Bindings
- Counter-Possession
- Counter-Subversion
- Information Technology
- Media Relations
- Medical and Psychological
- Occult Forensics
- Plumber (that being what the Laundry call their paramilitary clean-up operatives)
Wealth Levels instead of specific savings and income.
Some changes to skill list
The big mechanical change is apparently a new magic system. It covers traditional/ritual magic, Laundry-style computational magic, and "mental" magic (which is casting spells without external aid). As noted above, Magic Points are out, but casting now requires a skill check with a Sorcery skill.
Combat and investigation are streamlined, although I haven't seen anything about exactly how. Also, nice things were said about the sanity system.
Special Successes (rolling under 1/5 of your skill) replace criticals/impales. On attack rolls, they do double damage. Outside of combat, I think they're just extra good successes without any special rules.
Best Answer
I run into this issue all the time with GURPS. The basic assumptions of GURPS/BRP rules are different than that of the AD&D rules. Unless you are allowing the equivalent of superpowers there is no time that a GURPS/BRP is worth four "normal" characters like a 4th level D&D fighter.
So are two routes you can take. First you can have BRP (or GURPS) simulate D&D dungeon fantasy with the characters able to withstand increase damage. GURPS had dungeon fantasy that starts characters off at 250 pt and in theory BRP can handle it the same way. (Not sure what combination of books will get you there).
The second is to use a regular fantasy character of the system and adapt the module. Most players and referee, including myself, like doing this because these type of characters represent what we like about the system (GURPS, BRP, etc) in the first place.
So how you adapt the module? Well one way is to look at it realistically. Imagine a world where Hill Giant existed. They would have a society with a range of skills and aptitudes. So the first thing is don't have every hill giant be a skilled fighter. This plays nicely into the skill based system of BRP. So instead of the 12 fearsome Hill Giant warriors of the AD&D original you have two good ones, five that are ok, and the rest are mediocre. Note the original Giant modules were good about including non-combatants like the wives, and children. So they they are well suited for this treatment. So yes the Hill Giants are damn tough but now the highly skilled PCs have a excellent chance of besting them due to superior skill.
Another thing to do is flesh out the locale in the module especially the NPCs. This will provide additional methods of dealing with the situations through roleplay and plans. One of the reason many adopt BRP and GURPS is that character can be good at other things than swords and spells. Create opportunities to do those other thing.
Beyond this you need to be talking about the specific modules. I can try to help if you open another question about which module you are trying to convert. I have adapted several to use with GURPS so I have some experience doing this.