While Oroniss summarised Seelah pretty well in my opinion, the Paladin is my favorite Class since I started playing ad&d 2nd Edition 20 somewhat years ago, so I try to add some of my thoughts:
I played Seelah solo through Perils, Burnt Offerings and Skinsaw Murders and started her in a 6 Character Group with Lini, Harsk, Merisiel, Ezren and Seoni.
For Skill-Feats I went in both Cases with her strengths (Strength first then Wisdom and Strength again) and left the weakspots Dexterity and Intelligence to Allies or other Characters.
Power Feats I prefer to go with improving the bonus d6 first (spell recharge befor +1).
I don't think her limited handsize is as troublesome as for some other Characters like Sajan.
With Card Feats I feel the second Spell is a good first Choice and I feel Allies are always a good Choice for their flexibility, but I tend to make my second card choice more based on the problems that came up during the last few Scenarios.
For Stategies:
Divine Gace
Don't be afraid to use your bonus d6. 40% of your Cards will be recharged and with the Spell upgrade you can easily get half your Deck rechargeble. As soon as you have a solid hand it will cut out the ballast and turn into a 100% recharge cycle.
Detect Evil
The Bonus Scouting is really powerfull. While I don't agree to use it every Round it helps you to cut your exploration Time down quite a bit.
I tend to use it in certain Locations (Garrison/ Shrine), to get an advantage over the Blessings Deck, or when I feel confident in the Equipment of my group.
Plus since you use it at the Start (or End) of your Turn you can move away, if the Top-Card is an early unwanted Villain.
I tend to use it more in bigger Group Games than Solo Games for the simple reason that you have to make the most out of every turn in bigger Games.
Building your Deck:
I feel getting magical Armor (not necessarily Shields) is the most important thing for Seelah, even if they are only light and if you can convince the other players to give them to you. All light Armor Classes have a way to turn unwanted Handcards into something usefull, but Seelah not only need one Armor in her Starthand, her only way to get rid of unwanted extras, that clot your hand, is to take voluntary Damage. Beeing able to recharge them at the end of your round will do wonders for your limited Hand-Size.
For spells I prefer to start with a basic Detection Spell and aim to trade it in for Find Traps as soon as possible. It might give you only more of your Basic dies, but with your other possible bonusses this will add up quickly. For the second Spell i tend to start with Cure if available, until I get a feeling for the changes in difficulty.
Generally I try to keep Seelah to Divinations and leave support and combat spells to the Cleric/Bard/Druid.
For Allies I prefer the Sage and the Troubadour since they offer a d6 Bonus to Seelahs d4-Skills.
Having seen the HMM-Cards I feel Seelahs biggest weakness is going to be her inability to use Items. The number of Cards that give more permanent or reusable Bonusses startet strong and got even better with every Adventure Deck. And with Seelahs already average Skills I fear she might loose even more, if bigger Bonusses come around in the secend half. Her Divine Gace is good but can compensate only so much.
